Reporting more CPU features in /proc/cpuinfo

[Mark Rutland]

On Mon, Apr 23, 2018 at 08:20:43AM -0600, Jeffrey Hugo wrote:
> On 4/20/2018 2:04 PM, Kees Cook wrote:
> > Hi!
> > 
> > I'd like to have more CPU features visible in /proc/cpuinfo so it's
> > possible to determine from userspace if the running system has various
> > defenses. For example, I'd like to know that a device has hardware PXN
> > and PAN. (I'd love to see emulated features listed separately too, but
> > I could imagine that not being a great match for /proc/cpuinfo.)
> > 
> > One specific reason for this to do basic feature testing from
> > userspace without needing to catch the boot-time dmesg output. For
> > example, being able to answer the question "Does this system provide
> > PAN?" The answer is yes if either emulated PAN is visible in
> > /proc/config.gz or if hw PAN Is provided by the CPU (which I'd expect
> > to discover via /proc/cpuinfo like on x86 for things like SMAP).
> > 
> > Thoughts?
> > 
> > -Kees
> 
> I too, would like to see /proc/cpuinfo to be more useful, but last time this
> was brought up by Al Stone[1], there was significant pushback from Mark.
> 
> Maybe Mark has changed his position since then?
> 
> [1] https://lkml.org/lkml/2017/9/26/786

The big deal is that we cannot change the format of /proc/cpuinfo, i.e.
no new fields, or changes to the format of existing fields. Nothing has
changed there.

It's fine to add new hwcaps, so the question is whether PXN and PAN
should have hwcaps. It would be rather unusual, especially for SW PAN,
but that's not a hard NAK from me.

Kees, when does this information matter? e.g. is this some compliance
test, or something that would run at arbitrary times on a given system?

Thanks,
Mark.