Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 28 Aug 2020 20:42:09 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v2] Documentation/x86: Add documentation for /proc/cpuinfo feature flags |
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On Mon, Aug 24, 2020 at 11:04:12AM -0700, Kyung Min Park wrote: > +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, the users may use > +tools like http://www.etallen.com/cpuid.html (which is not updated with > +kernel releases) or other custom tools that read CPUID.
I guess this should talk only about our own kcpuid tool since we wanna do that now, right?
...
> +c: The kernel disabled support for it at compile-time. > +------------------------------------------------------ > +For example, if 5-level-paging is not enabled when building (i.e., > +CONFIG_X86_5LEVEL is not selected) the flag "la57" will not show up [#f1]_. > +Even though the feature will still be detected via CPUID, the kernel disables > +it via cleared by setup_clear_cpu_cap(X86_FEATURE_LA57).
"... disables it by clearing... "
> +d: The feature is disabled at boot-time. > +---------------------------------------- > +A feature can be disabled either using a command-line parameter or because > +it failed to be enabled. The command-line parameter clearcpuid= can be used > +to disable features using the feature number as defined in > +/arch/x86/include/asm/cpufeatures.h. For instance, User Mode Instruction > +Protection can be disabled using clearcpuid=514. The number 514 is calculated > +from #define X86_FEATURE_UMIP (16*32 + 2). > + > +In addition, there exists a variety of custom command-line parameters that > +disable specific features. The list of parameters includes, but is not limited > +to, no5lvl, nosmap, and nosmep.
You already give the separate example for "no5lvl" below so use something else above, say, "nofsgsbase", for example.
> 5-level paging can also be disabled using > +"no5lvl". SMAP and SMEP are disabled with the aforementioned parameters, > +respectively. > + > +e: The feature was known to be non-functional. > +---------------------------------------------- > +The feature was known to be non-functional because a dependency was > +missing at runtime. For example, AVX flags will not show up if XSAVE feature > +is disabled since they depend on XSAVE feature.
Another example would be: broken CPUs and them missing microcode patches and due to that the kernel deciding not to enable a feature.
But yap, all in all looks like a good idea. I'll take the next version after you've fixed those nitpicks.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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