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Documentation/x86: Document what /proc/cpuinfo is for

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This has been long overdue. Write down what x86's version of
/proc/cpuinfo is and should be used for.

With improvements by dhansen.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20231129101700.28482-1-bp@alien8.de
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Borislav Petkov (AMD) 2023-12-08 22:57:33 +01:00
parent a24d61c609
commit 79c603ee43
1 changed files with 66 additions and 19 deletions
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Documentation/arch/x86/cpuinfo.rst
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@ -7,27 +7,74 @@ x86 Feature Flags
Introduction
============
On x86, flags appearing in /proc/cpuinfo have an X86_FEATURE definition
in arch/x86/include/asm/cpufeatures.h. If the kernel cares about a feature
or KVM want to expose the feature to a KVM guest, it can and should have
an X86_FEATURE_* defined. These flags represent hardware features as
well as software features.
The list of feature flags in /proc/cpuinfo is not complete and
represents an ill-fated attempt from long time ago to put feature flags
in an easy to find place for userspace.
If users want to know if a feature is available on a given system, they
try to find the flag in /proc/cpuinfo. If a given flag is present, it
means that the kernel supports it and is currently making it available.
If such flag represents a hardware feature, it also means that the
hardware supports it.
However, the amount of feature flags is growing by the CPU generation,
leading to unparseable and unwieldy /proc/cpuinfo.
What is more, those feature flags do not even need to be in that file
because userspace doesn't care about them - glibc et al already use
CPUID to find out what the target machine supports and what not.
And even if it doesn't show a particular feature flag - although the CPU
still does have support for the respective hardware functionality and
said CPU supports CPUID faulting - userspace can simply probe for the
feature and figure out if it is supported or not, regardless of whether
it is being advertised somewhere.
Furthermore, those flag strings become an ABI the moment they appear
there and maintaining them forever when nothing even uses them is a lot
of wasted effort.
So, the current use of /proc/cpuinfo is to show features which the
kernel has *enabled* and *supports*. As in: the CPUID feature flag is
there, there's an additional setup which the kernel has done while
booting and the functionality is ready to use. A perfect example for
that is "user_shstk" where additional code enablement is present in the
kernel to support shadow stack for user programs.
So, if users want to know if a feature is available on a given system,
they try to find the flag in /proc/cpuinfo. If a given flag is present,
it means that
* the kernel knows about the feature enough to have an X86_FEATURE bit
* the kernel supports it and is currently making it available either to
userspace or some other part of the kernel
* if the flag represents a hardware feature the hardware supports it.
The absence of a flag in /proc/cpuinfo by itself means almost nothing to
an end user.
On the one hand, a feature like "vaes" might be fully available to user
applications on a kernel that has not defined X86_FEATURE_VAES and thus
there is no "vaes" in /proc/cpuinfo.
On the other hand, a new kernel running on non-VAES hardware would also
have no "vaes" in /proc/cpuinfo. There's no way for an application or
user to tell the difference.
The end result is that the flags field in /proc/cpuinfo is marginally
useful for kernel debugging, but not really for anything else.
Applications should instead use things like the glibc facilities for
querying CPU support. Users should rely on tools like
tools/arch/x86/kcpuid and cpuid(1).
Regarding implementation, flags appearing in /proc/cpuinfo have an
X86_FEATURE definition in arch/x86/include/asm/cpufeatures.h. These flags
represent hardware features as well as software features.
If the kernel cares about a feature or KVM want to expose the feature to
a KVM guest, it should only then expose it to the guest when the guest
needs to parse /proc/cpuinfo. Which, as mentioned above, is highly
unlikely. KVM can synthesize the CPUID bit and the KVM guest can simply
query CPUID and figure out what the hypervisor supports and what not. As
already stated, /proc/cpuinfo is not a dumping ground for useless
feature flags.
If the expected flag does not appear in /proc/cpuinfo, things are murkier.
Users need to find out the reason why the flag is missing and find the way
how to enable it, which is not always easy. There are several factors that
can explain missing flags: the expected feature failed to enable, the feature
is missing in hardware, platform firmware did not enable it, the feature is
disabled at build or run time, an old kernel is in use, or the kernel does
not support the feature and thus has not enabled it. In general, /proc/cpuinfo
shows features which the kernel supports. For a full list of CPUID flags
which the CPU supports, use tools/arch/x86/kcpuid.
How are feature flags created?
==============================