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sysarch.x86.2: manual page for the sysarch(2) functions on x86
Reviewed by: ziaee Discussed with: jhb Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D60047
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@@ -72,7 +72,8 @@ for information about their return values.
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.Sh SEE ALSO
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.Xr i386_get_ioperm 2 ,
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.Xr i386_get_ldt 2 ,
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.Xr i386_vm86 2
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.Xr i386_vm86 2 ,
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.Xr sysarch.x86 2
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.Sh HISTORY
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This manual page was taken from
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.Nx .
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@@ -7,7 +7,8 @@ SRCS+= \
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.if ${LIB} == "sys"
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MAN+= \
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pkru.3
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pkru.3 \
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sysarch.x86.2
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.endif # ${LIB} == "sys"
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# Note: vdso support for hyperv only on amd64
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@@ -0,0 +1,659 @@
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.\" SPDX-License-Identifier: BSD-2-Clause
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.\"
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.\" Copyright 2026 The FreeBSD Foundation.
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.\"
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.\" This documentation was written by Konstantin Belousov <[email protected]>
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.\" under sponsorship of The FreeBSD Foundation.
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.\"
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.Dd September 26, 2026
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.Dt SYSARCH.X86 2 i386
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.Os
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.Sh NAME
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.Nm sysarch.x86
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.Nd architecture-dependent system call on x86
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In machine/sysarch.h
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.Ft int
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.Fn sysarch "int number" "void *args"
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.Sh DESCRIPTION
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The
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manual page describes the functions provided by the
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.Fn sysarch
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system call on x86 architectures, that is i386 and amd64, see
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.Xr arch 7 .
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.Ss Parameter Types
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The following structures are used as the arguments pointed to by the
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.Fa args
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parameter.
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Refer to the description of each request for the meaning of
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individual fields:
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.Pp
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The
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.Dv I386_GET_IOPERM
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and
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.Dv I386_SET_IOPERM
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requests take a
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.Vt struct i386_ioperm_args ,
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which is defined as follows:
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.Bd -literal
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struct i386_ioperm_args {
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unsigned int start;
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unsigned int length;
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int enable;
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};
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.Ed
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.Pp
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The
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.Dv I386_GET_LDT
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and
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.Dv I386_SET_LDT
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requests take a
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.Vt struct i386_ldt_args ,
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whose definition differs between i386 and amd64.
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The i386 variant is:
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.Bd -literal
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struct i386_ldt_args {
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unsigned int start;
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union descriptor *descs;
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unsigned int num;
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};
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.Ed
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The amd64 variant is:
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.Bd -literal
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struct i386_ldt_args {
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unsigned int start;
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struct user_segment_descriptor *descs __packed;
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unsigned int num;
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};
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.Ed
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.Pp
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The request
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.Dv I386_VM86
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is only supported by i386 kernels and takes a
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.Bd -literal
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struct i386_vm86_args {
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int sub_op;
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char *sub_args;
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};
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.Ed
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.Pp
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The
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.Dv I386_GET_XFPUSTATE ,
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takes a
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.Vt struct i386_get_xfpustate
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argument, a
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.Dv AMD64_GET_XFPUSTATE
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.Vt struct amd64_get_xfpustate
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argument.
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The structures have the same members, but their layout differs due to
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pointer size differences.
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Only the amd64 variant is listed for clarity.
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.Bd -literal
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struct amd64_get_xfpustate {
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void *addr;
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int len;
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};
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.Ed
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.Pp
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The
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.Dv I386_GET_PKRU ,
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.Dv I386_SET_PKRU ,
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take a
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.Vt struct i386_set_pkru
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argument, and
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.Dv AMD64_GET_PKRU
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and
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.Dv AMD64_SET_PKRU
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take a
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.Vt struct amd64_set_pkru
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argument.
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The structures have the same members, but their layout differs due to
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pointer size differences.
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Only the amd64 variant is listed for clarity.
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.Bd -literal
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struct amd64_set_pkru {
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void *addr;
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unsigned long len;
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unsigned int keyidx;
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int flags;
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};
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.Ed
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.Ss Requests
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The following requests can be specified by the
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.Fa number
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argument:
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.Bl -tag -width AMD64_DISABLE_TLSBASE
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.It Dv I386_GET_LDT
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Queries the specified range of LDT descriptors for the
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calling process.
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.Pp
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The request takes a pointer to the
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.Vt struct i386_ldt_args
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structure as the
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.Fa args
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parameter.
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Its
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.Va start
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member specifies the starting index, and
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.Va num
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the number of descriptors which are queried.
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The LDT descriptors in the requested range are copied into
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the memory specified by the
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.Fa descs
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member.
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.It Dv I386_SET_LDT
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Set the specified range of LDT descriptors for the calling process.
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.Pp
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LDT is process-global, all process threads share the same table.
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On the other hand, if several processes share an address space,
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they still each have their own LDT.
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.Pp
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The request takes a pointer to the
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.Vt struct i386_ldt_args
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structure as the
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.Fa args
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parameter.
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Its
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.Va start
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member specifies the starting index, and
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.Va num
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the number of descriptors which are set.
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The LDT descriptors in the requested range
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are initialized from the structures pointed to by the
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.Fa descs
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member.
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.Pp
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If the
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.Va descs
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member value is
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.Dv NULL,
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the LDT descriptors are reset to null descriptors.
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If the
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.Va start
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starting descriptor index has the special value
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.Dv LDT_AUTO_ALLOC
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and the number of descriptors to allocate is 1, then the system looks for
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a free descriptor index and allocates it.
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The resulting index is returned as the system call return value.
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.Pp
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Only memory read or read/write normal or expand down, and execute only
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or execute read segment types are allowed.
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Any system segments descriptors or memory conforming execute segments
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are rejected with the
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.Ev EACCESS
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error.
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.Pp
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X86 hardware supports LDTs with up to 8192 descriptors.
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This size is excessive for modern applications.
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The
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.Dv machdep.max_ldt_segment
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loader tunable sets a system-global maximum LDT size
|
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(in number of descriptors).
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.It Dv I386_GET_IOPERM
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Queries the state of userspace access to the specified range of
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I/O ports for the calling thread.
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.Pp
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The function takes a
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.Vt struct i386_ioperm_args
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structure as the
|
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.Fa args
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argument.
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The
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.Va start
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member is the starting port number to query.
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Upon successful return, the
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.Va enable
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||||
member records the access permissions for the starting port.
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The
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.Va length
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member indicates how many ports after the starting port number
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have the same access permissions.
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.Pp
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||||
For instance, if
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||||
.Va start
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was set to 10,
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and the request set the
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.Va length
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field to 3, with
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.Va enable
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set to 1, then access to ports 10, 11, and 12 is allowed, while
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access to port 13 is disallowed.
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.It Dv I386_SET_IOPERM
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Grants or revokes userspace access to I/O ports by modifying the TSS
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I/O permission bitmap for the calling thread.
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Each thread maintains a separate bitmap.
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.Pp
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The function takes a
|
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.Vt struct i386_ioperm_args
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structure as the
|
||||
.Fa args
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argument.
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Depending on the
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.Va enable
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member value, it either grants (enables) or revokes (disables)
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access to the range of
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I/O ports specified by the
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.Va start
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and
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.Va length
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members.
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.Pp
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There is total of 65536 ports, so the set of ports specified must fit
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into the 0..65535 range.
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.Pp
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The call requires super-user privileges.
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.It Dv I386_VM86
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Execute CPU instructions in virtual 8086 (VM86) mode.
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This function is only supported by i386 kernels as VM86 mode
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is not supported in long mode.
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To use this request, the additional header
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.In machine/vm86.h
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must be included which exists only for i386.
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.Pp
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The request takes a
|
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.Vt struct i386_vm86_args
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||||
structure as the
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||||
.Fa args
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||||
argument.
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||||
Its
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.Va sub_op
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member specifies one of the following sub-requests:
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.Pp
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.Bl -tag -width VM86_SET_VME
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.It Dv VM86_INIT
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Initialize the VM86 in-kernel extended pcb.
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Without this initialization, VM86 mode cannot be entered.
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.Pp
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||||
The sub-request takes the
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||||
.Vt struct vm86_init_args
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||||
additional argument pointed to by the
|
||||
.Fa sub_args
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||||
pointer in the
|
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.Vt struct i386_vm86_args
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||||
structure.
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||||
.Pp
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||||
Currently only the
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||||
.Va int_map
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member of the structure is implemented.
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||||
Each bit N in the
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||||
.Va int_map
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||||
bit-string set to 1 causes a
|
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.Dv SIGTRAP
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signal to be delivered to the thread on the execution
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of the
|
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.Dv INT N
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||||
instruction.
|
||||
Otherwise, if the bit is zero, the execution of the
|
||||
.Dv INT N
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||||
instruction is reflected to VM86 mode to the interrupt
|
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handler as designated by the real-mode interrupt table.
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.It Dv VM86_SET_VME
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Reserved but not implemented.
|
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.It Dv VM86_GET_VME
|
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Returns the state of the VME extensions.
|
||||
The
|
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.Va sub_args
|
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must point to a
|
||||
.Vt struct vm86_vme_args
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||||
structure.
|
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.It Dv VM86_INTCALL
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Arrange execution in VM86 mode as if the specified interrupt
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was delivered.
|
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Takes a
|
||||
.Vt struct vm86_intcall_args
|
||||
argument as the
|
||||
.Va sub_args
|
||||
pointer.
|
||||
The
|
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.Va intnum
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||||
member of it provides the interrupt number to emulate delivery of,
|
||||
and the
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.Va vmf
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||||
gives the VM86 mode machine context to load for the interrupt
|
||||
handler.
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||||
.El
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.Pp
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To switch thread to the VM86 execution mode, several preparations
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||||
must be done:
|
||||
.Bl -dash
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.It
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An alternative signal stack must be created, since otherwise signal
|
||||
delivery in VM86 mode would try to use VM86 stack.
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See
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.Xr sigaltstack 2 .
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.It
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The VM86 mode kernel state must be initialized with the
|
||||
.Dv VM86_INIT
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sub-request.
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.It
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The memory must be mapped in the first megabyte of the user address
|
||||
space, which will be accessible to the VM86 mode execution.
|
||||
.It
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Signal handlers must be established that can handle VM86 exceptions.
|
||||
Most importantly, the
|
||||
.Dv SIGSEGV
|
||||
handler must be provided to assist VM86 code to handle exits due to
|
||||
IOPL-sensitive instructions.
|
||||
.It
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Finally, the VM86 mode can be entered by switching to the context
|
||||
with %eflags register having the
|
||||
.Dv eflags.VM
|
||||
bit set.
|
||||
This can be done, among other ways, by loading the full
|
||||
machine context with the
|
||||
.Xr sigreturn 2
|
||||
system call.
|
||||
.El
|
||||
.Pp
|
||||
The thread continues to execute in VM86 mode until a signal
|
||||
is delivered to it.
|
||||
Note that exceptions generate appropriate signals the same as
|
||||
for protected execution mode.
|
||||
.Pp
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||||
See
|
||||
.Pa src:tools/tests/vm86
|
||||
for a minimal example for a VM86 monitor and sample code.
|
||||
.It Dv I386_GET_FSBASE
|
||||
Read the 32-bit fsbase context value for the calling thread into a
|
||||
.Vt uint32_t
|
||||
variable pointed to by the
|
||||
.Fa args
|
||||
argument.
|
||||
.It Dv I386_SET_FSBASE
|
||||
Set the 32-bit fsbase context value for the calling thread.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to a
|
||||
.Vt int32_t
|
||||
variable that has the new fsbase value stored.
|
||||
.Pp
|
||||
The fsbase is maintained by writing the fsbase value into the
|
||||
base field of the user 32-bit %fs GDT descriptor, which has index 2.
|
||||
.It Dv I386_GET_GSBASE
|
||||
Read the 32-bit gsbase context value for the calling thread into a
|
||||
.Vt int32_t
|
||||
variable pointed to by the
|
||||
.Fa args
|
||||
argument.
|
||||
.It Dv I386_SET_GSBASE
|
||||
Set the 32-bit gsbase context value for the calling thread.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to a
|
||||
.Vt int32_t
|
||||
variable that has the new gsbase value stored.
|
||||
.Pp
|
||||
The gsbase is maintained by writing the gsbase value into the
|
||||
base field of the user 32-bit %gs GDT descriptor, which has index 3.
|
||||
.It Dv I386_GET_XFPUSTATE
|
||||
Same as
|
||||
.Dv AMD64_GET_XFPUSTATE ,
|
||||
but takes the address of the
|
||||
.Vt struct i386_get_xfpustate
|
||||
structure as the
|
||||
.Fa args
|
||||
argument.
|
||||
.It Dv I386_SET_PKRU
|
||||
Provided only when running i386 binaries on the amd64 host kernel.
|
||||
.Pp
|
||||
See description of the
|
||||
.Dv AMD64_SET_PKRU
|
||||
request.
|
||||
.It Dv I386_CLEAR_PKRU
|
||||
Provided only when running i386 binaries on the amd64 host kernel.
|
||||
.Pp
|
||||
See description of the
|
||||
.Dv AMD64_CLEAR_PKRU
|
||||
request.
|
||||
.It Dv I386_SET_SPLITLOCK
|
||||
Change the requesting thread splitlock detection control.
|
||||
For i386 binaries, the function is only available when running
|
||||
on the amd64 host kernel.
|
||||
.Pp
|
||||
The argument must point to the 32bit
|
||||
.Vt int
|
||||
variable, holding one of the following values:
|
||||
.Bl -tag
|
||||
.It Dv 0
|
||||
Disable detection of split locking.
|
||||
.It Dv 1
|
||||
Enable detection of split locking.
|
||||
.El
|
||||
.Pp
|
||||
The split lock is the situation where the read-modify-write atomic
|
||||
instruction is performed on the memory location that is split over
|
||||
more than one CPU cache line.
|
||||
Some CPUs can be configured to generate the Alignment Fault exception
|
||||
.Ql #AC
|
||||
if split locking is detected.
|
||||
The request makes it possible for the thread to control the detector.
|
||||
.Pp
|
||||
The
|
||||
.Xr _umtx_op 2
|
||||
operations that pass unaligned locks to kernel requests returns
|
||||
.Ev EFAULT
|
||||
when splitlock detection is enabled.
|
||||
.Pp
|
||||
Presence of the detector can be queried with the
|
||||
.Dv hw.splitlock
|
||||
.Xr sysctl 2
|
||||
MIB.
|
||||
.It Dv I386_GET_SPLITLOCK
|
||||
Returns the state of the splitlock detection control.
|
||||
For i386 binaries, the function is only available when running
|
||||
on the amd64 host kernel.
|
||||
.Pp
|
||||
The argument must point to the 32bit
|
||||
.Vt int
|
||||
variable.
|
||||
The function writes the value 0 if the control is disabled
|
||||
and 1 if enabled.
|
||||
.Pp
|
||||
.It Dv AMD64_GET_FSBASE
|
||||
Returns the value of the
|
||||
.Dv fsbase
|
||||
register for the calling thread.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to the 64bit
|
||||
.Vt unsigned long variable
|
||||
where the value is written.
|
||||
.Pp
|
||||
On CPUs that do not support the
|
||||
.Dv RDFSBASE
|
||||
instruction, the call is the only
|
||||
way to read the current value of fsbase.
|
||||
.It Dv AMD64_SET_FSBASE
|
||||
Sets the
|
||||
.Dv fsbase
|
||||
register to the 64bit value pointed to by the
|
||||
.Fa args
|
||||
argument.
|
||||
.Pp
|
||||
The value is checked to not point outside of the allowed user address
|
||||
space, which is subset of possible canonical addresses.
|
||||
If the
|
||||
.Dv WRFSBASE
|
||||
instruction is supported, the fsbase register can
|
||||
be updated either by the call or by the instruction.
|
||||
For 64bit processes, the fsbase register is always restored on return
|
||||
to user mode from kernel calls or interrupts, regardless of the value
|
||||
of the %fs segment register.
|
||||
.It Dv AMD64_GET_GSBASE
|
||||
Get the
|
||||
.Dv gsbase
|
||||
register, see description of
|
||||
.Dv AMD64_GET_FSBASE .
|
||||
.It Dv AMD64_SET_GSBASE
|
||||
Set the
|
||||
.Dv gsbase
|
||||
register, see description of
|
||||
.Dv AMD64_GET_GSBASE .
|
||||
.It Dv AMD64_GET_XFPUSTATE
|
||||
Fills the extended FPU state as defined by the architecture and the
|
||||
running CPU hardware into the memory described by the
|
||||
.Vt struct amd64_get_xfpustate
|
||||
structure.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to the structure, the extended state (that is, full
|
||||
XSAVE area excluding the legacy region) is written to the
|
||||
.Va addr
|
||||
up to the
|
||||
.Va len
|
||||
bytes.
|
||||
If the
|
||||
.Va len
|
||||
member value is smaller then the XSAVE area size, the result is truncated.
|
||||
.It Dv AMD64_SET_PKRU
|
||||
Set the 4bit protection key for the page table entries used to map
|
||||
the given userspace virtual address range.
|
||||
.Pp
|
||||
The request takes the
|
||||
.Vt struct amd64_set_pkru
|
||||
structure pointed to by the
|
||||
.Fa args
|
||||
parameter.
|
||||
The
|
||||
.Va addr
|
||||
and
|
||||
.Va len
|
||||
members of the structure specify starting address and the length in bytes
|
||||
of the region.
|
||||
The
|
||||
.Va keyidx
|
||||
member value must be between 0 and 15 and provides the protection key
|
||||
index applied to the region.
|
||||
The
|
||||
.Va flags
|
||||
member is or-ed with the following flags:
|
||||
.Bl -tag -width AMD64_PKRU_PERSIST
|
||||
.It Dv AMD64_PKRU_EXCL
|
||||
Requires that no protection key was assigned to the specified range,
|
||||
or previously assigned key cleared.
|
||||
If some key was already applied to any part of the range, the call
|
||||
fails with the
|
||||
.Ev EBUSY
|
||||
error.
|
||||
.It Dv AMD64_PKRU_PERSIST
|
||||
Makes the assigned key persistent.
|
||||
It is automatically re-applied to any new mapping created in the range.
|
||||
.El
|
||||
.Pp
|
||||
Protection keys feature is optional and depends on CPU implementing it.
|
||||
Use the
|
||||
.Dv CPUID
|
||||
instruction to check its availability.
|
||||
.Pp
|
||||
The request is used to implement the
|
||||
.Xr x86_pkru_protect_range 3
|
||||
function.
|
||||
.It Dv AMD64_CLEAR_PKRU
|
||||
Clear any protection keys assigned to the specified region.
|
||||
.Pp
|
||||
The request takes the
|
||||
.Vt struct amd64_set_pkru
|
||||
structure pointed to by the
|
||||
.Fa args
|
||||
parameter.
|
||||
The
|
||||
.Va addr
|
||||
and
|
||||
.Va len
|
||||
members of the structure specify starting address and the length in bytes
|
||||
of the region.
|
||||
Other structure members are ignored.
|
||||
.Pp
|
||||
The request is used to implement the
|
||||
.Xr x86_pkru_unprotect_range 3
|
||||
function.
|
||||
.It Dv AMD64_GET_TLSBASE
|
||||
AMD64 only.
|
||||
.Pp
|
||||
Returns the current TLSBASE value for the calling thread.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to the 64bit
|
||||
.Vt unsigned long
|
||||
variable where the value is written to.
|
||||
.It Dv AMD64_SET_TLSBASE
|
||||
Sets the TLSBASE and the fsbase register values for the calling thread.
|
||||
The
|
||||
.Fa args
|
||||
argument must point to the 64bit
|
||||
.Vt unsigned long
|
||||
variable where the value is taken from.
|
||||
.Pp
|
||||
The TLSBASE value is written into the
|
||||
.Dv fsbase
|
||||
register before a signal handler is invoked.
|
||||
The current fsbase value is saved, and then restored
|
||||
upon return from the signal handler.
|
||||
Use of TLSBASE makes it possible for programs that modify
|
||||
the fsbase register using the
|
||||
.Dv WRFSBASE
|
||||
instruction to reliably access
|
||||
thread-local variables in signal handlers.
|
||||
.Pp
|
||||
TLSBASE is set up by the
|
||||
.Fx
|
||||
C runtime environment on thread startup.
|
||||
.It Dv AMD64_DISABLE_TLSBASE
|
||||
Disables loading the fsbase register from TLSBASE value on
|
||||
a signal handler invocation.
|
||||
The argument
|
||||
.Fa args
|
||||
is ignored.
|
||||
.Pp
|
||||
The call allows to undo the effect of the
|
||||
.Dv AMD64_SET_TLSBASE ,
|
||||
making signal handlers retain the value of fsbase from the
|
||||
interrupted context.
|
||||
By default, tlsbase is disabled after the
|
||||
.Xr execve 2
|
||||
syscall activated a new image.
|
||||
.El
|
||||
.Sh RETURN VALUES
|
||||
For all requests except
|
||||
.Dv I386_GET_LDT
|
||||
and
|
||||
.Dv I386_SET_LDT ,
|
||||
upon successful completion, the value 0 is returned; otherwise the
|
||||
value -1 is returned and the global variable
|
||||
.Va errno
|
||||
is set to indicate the error.
|
||||
.Pp
|
||||
The
|
||||
.Dv I386_SET_LDT
|
||||
request returns the starting index of the descriptors range it operated
|
||||
on, on success.
|
||||
In particular, the index of the automatically allocated descriptor is
|
||||
reported this way.
|
||||
Otherwise, -1 is returned and
|
||||
.Va errno
|
||||
is set.
|
||||
.Pp
|
||||
The
|
||||
.Dv I386_GET_LDT
|
||||
request returns 0 if the specified descriptors range is completely
|
||||
outside the allocated LDT.
|
||||
On any other error, -1 is returned and
|
||||
.Va errno
|
||||
is set.
|
||||
Otherwise the number of reported descriptors is returned as the syscall
|
||||
return value.
|
||||
.Sh SEE ALSO
|
||||
.Xr i386_get_ioperm 2 ,
|
||||
.Xr i386_get_ldt 2 ,
|
||||
.Xr i386_vm86 2 ,
|
||||
.Xr sysarch 2 ,
|
||||
.Xr pkru 3 ,
|
||||
.Xr arch 7
|
||||
.Sh HISTORY
|
||||
The manual page appeared in
|
||||
.Fx 16.0 .
|
||||
.Sh AUTHORS
|
||||
The manual page was written by
|
||||
.An Konstantin Belousov Aq Mt [email protected] .
|
||||
Reference in new issue
Block a user