Re: [PATCH v2] selftests/x86: Add a test for LASS enforcement

[Sohil Mehta <sohil.mehta@intel.com>]

On 10/13/2026 11:54 AM, H. Peter Anvin wrote:
> On 2026-10-08 19:00, Sohil Mehta wrote:
>>
>> Note, the CPUID bit does not reflect whether the kernel enabled LASS;
>> only the cpuinfo flag does. But parsing cpuinfo is cumbersome, and the
>> kernel enables LASS whenever the CPU supports it unless the command line
>> opts out (vsyscall=emulate or clearcpuid=lass). To keep it simple, rely
>> on the CPUID bit and accept a failure in these unusual cases.
>>
> Cumbersome, really?
> 

I had it in v1 of this patch. It seemed excessive to do specifically for
LASS:
https://lore.kernel.org/lkml/20260806011536.4172258-8-sohil.mehta@intel.com/

As LASS is always enabled by the kernel except for the really odd cases
the trade-off seems practical to me.

> Here is a set of parsing functions -- tested, even! -- that you can make
> available as library routines. Integrating them into the selftests build
> framework has been left as an exercise for the reader ;-)
> 

I agree, adding them as generic selftest helpers may be useful. But,
/proc/cpuinfo has been unreliable in general. It is currently buggy for
some features with Kconfig options. See:

https://lore.kernel.org/lkml/f6f113b46199d996c4f81fc89858c2f484256418.1774623092.git.m.wieczorretman@pm.me/

Also, (I think) we have been generally discouraging folks to add more
bits there. So, I don't know if creating these library routines is
useful right now. IIUC, Boris said that the CPUID rework would make some
of cpuinfo stuff moot.

https://lore.kernel.org/lkml/20260331140216.GDacvT6IPW1gf003z-@fat_crate.local/

Thoughts?