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From: Keno Fischer <>
Subject: [PATCH] arch_prctl.2: Add ARCH_SET_CPUID subcommand
Date: Sun, 15 Mar 2020 23:21:27 -0400
Message-ID: <> (raw)

This subcommand was added a few years ago to support cpuid emulation
on x86 targets, but no changes to the man page appear to have been
made at the time. This commit adds a description for it and the
corresponding getter.

Signed-off-by: Keno Fischer <>
 man2/arch_prctl.2 | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/man2/arch_prctl.2 b/man2/arch_prctl.2
index c49f4e0..e40d6bb 100644
--- a/man2/arch_prctl.2
+++ b/man2/arch_prctl.2
@@ -48,7 +48,44 @@ for the "set" operations, or as an
 .IR "unsigned long\ *" ,
 for the "get" operations.
-Subfunctions for x86-64 are:
+Subfunctions for both x86 and x86-64 are:
+.BR ARCH_SET_CPUID " (since Linux 4.12)"
+Enable (
+.I addr != 0
+) or disable (
+.I addr == 0
+) the
+.I cpuid
+instruction for the current thread. The instruction
+is enabled by default. If disabled,
+any execution of a
+.I cpuid
+instruction will instead generate a
+signal. This feature can be used to emulate
+.I cpuid
+results that differ from what the underlying
+hardware would have produced (e.g. in a paravirtualization setting).
+This setting is preserved across
+.BR fork (2)
+.BR clone (2)
+but reset to default (i.e.
+.I cpuid
+enabled) on
+.BR execve (2)
+.BR ARCH_GET_CPUID " (since Linux 4.12)"
+Return the setting of the flag manipulated by
+as the result of the system call (1 for enabled, 0 for disabled).
+.I addr
+is ignored.
+Subfunctions for x86-64 only are:
 Set the 64-bit base for the
@@ -96,6 +133,10 @@ is not a valid subcommand.
 .I addr
 is outside the process address space.
+was requested, but the underlying hardware does not support CPUID faulting.
 .\" Man page written by Andi Kleen.

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