Re: [RFC PATCH v2] tools/x86: add kcpuid tool to show raw CPU features

[Arvind Sankar]

On Tue, Sep 22, 2020 at 10:10:24PM +0200, Borislav Petkov wrote:
> + AMD folks.
> 
> On Tue, Sep 22, 2020 at 01:27:50PM +0800, Feng Tang wrote:
> > End users frequently want to know what features their processor
> > supports, independent of what the kernel supports.
> > 
> > /proc/cpuinfo is great. It is omnipresent and since it is provided by
> > the kernel it is always as up to date as the kernel. But, it could be
> > ambiguous about processor features which can be disabled by the kernel
> > at boot-time or compile-time.
> > 
> > There are some user space tools showing more raw features, but they are
> > not bound with kernel, and go with distros. Many end users are still
> > using old distros with new kernels (upgraded by themselves), and may
> > not upgrade the distros only to get a newer tool.
> > 
> > So here arise the need for a new tool, which
> >   * Shows raw cpu features got from running cpuid
> >   * Be easier to obtain updates for compared to existing userspace
> >     tooling (perhaps distributed like perf)
> >   * Inherits "modern" kernel development process, in contrast to some
> >     of the existing userspace cpuid tools which are still being developed
> >     without git and distributed in tarballs from non-https sites.
> >   * Can produce output consistent with /proc/cpuinfo to make comparison
> >     easier.

Rather than a tool, would additional file(s) in, say,
/sys/devices/system/cpu/cpu<n> be nicer? They could show the raw CPUID
features, one file per leaf or sub-leaf, maybe even along with whether
they were disabled at boot-time.

> >   * Be in-kernel, could leverage kernel enabling, and even
> >     theoretically consume arch/x86/boot/cpustr.h so it could pick up
> >     new features directly from one-line X86_FEATURE_* definitions.

That's arch/x86/include/asm/cpufeatures.h right -- cpustr.h is generated
from that. The table there already has comments which could be extracted
as the one-line description.