Messages in this thread Patch in this message | | | Date | Tue, 28 Nov 2023 20:36:50 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/cpu: Update power flags |
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On Tue, Nov 28, 2023 at 08:25:40PM +0100, Thomas Weißschuh wrote: > > The beginning of Documentation/arch/x86/cpuinfo.rst tries to explain > > why. > > Isn't this introduction more about the cpuinfo "flags" fields?
That's why I said "tries to explain". See below - I've been meaning to write this for a long time now.
> power management: ts ttp tm hwpstate cpb eff_freq_ro [13] [14] [15]
Yeah, nothing cares about those - see below.
Thx.
--- diff --git a/Documentation/arch/x86/cpuinfo.rst b/Documentation/arch/x86/cpuinfo.rst index 08246e8ac835..95c7ebcc13a9 100644 --- a/Documentation/arch/x86/cpuinfo.rst +++ b/Documentation/arch/x86/cpuinfo.rst @@ -13,6 +13,29 @@ 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 /proc/cpuinfo list is not exhaustive and represents an ill-fated +attempt from long time ago to put feature flags in an easy to find place +for userspace. 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 even need to be in that file because +userspace doesn't even 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 do that and the hw supports CPUID faulting, userspace can +simply probe for the feature and figure out if it is supported or not + +* 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 there and ready to use. A perfect +example for that is "user_shstk". + 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.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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