Messages in this thread | | | Subject | Re: [PATCH v2] Documentation/x86: Add documentation for /proc/cpuinfo feature flags | From | Kyung Min Park <> | Date | Fri, 28 Aug 2020 15:30:32 -0700 |
| |
Hi Boris,
On Fri, 2020-08-28 at 20:42 +0200, Borislav Petkov wrote: > 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?
Should I mention the tool specifically although the tool is WIP? As you commented previously, should I use tools/arch/x86/tools/cpuid/cpuid as the future tool and its location?
Or do you want it to be mentioned in the future tense without specifying the tool name and location?
> > ... > > > +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... "
Sure, let me fix it.
> > > +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.
You're right. Let me change it.
> > > 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.
Thank you for the comment. I'll add that too.
> > But yap, all in all looks like a good idea. I'll take the next > version > after you've fixed those nitpicks. > > Thx.
Thanks Boris!
| |