Re: [PATCH] x86/MTRR: hook mtrr_bp_restore() back up

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.03.2025 13:48, Roger Pau Monné wrote:
> On Thu, Mar 27, 2025 at 01:30:44PM +0100, Jan Beulich wrote:
>> On 27.03.2025 12:38, Roger Pau Monné wrote:
>>> On Thu, Mar 27, 2025 at 12:20:47PM +0100, Jan Beulich wrote:
>>>> Unlike stated in the offending commit's description,
>>>> load_system_tables() wasn't the only thing left to retain from the
>>>> earlier restore_rest_processor_state().
>>>>
>>>> While there also do Misra-related tidying for the function itself: The
>>>> function being used from assembly only means it doesn't need to have a
>>>> declaration, but wants to be asmlinkage.
>>>
>>> I wonder, maybe the intention was for the MTRR restoring on the BSP to
>>> also be done by the mtrr_aps_sync_end() call in enter_state()?
>>>
>>> AFAICT that will set the MTRRs uniformly on all CPUs, by calling
>>> mtrr_set_all() just like mtrr_bp_restore(), but later in the restore
>>> process.
>>
>> Hmm, yes, that's possible. The comment in set_mtrr() is somewhat misleading
>> then, though, as for the BP the writing then isn't just "okay" but necessary.
>> Question is whether doing this so much later is actually good enough.
> 
> Hm, no idea really.  We do the device restore ahead of the MTRR
> restore, so I wonder whether we could have issues by using unexpected
> effective cache attributes for device memory accesses as a result of
> MTRRs not being initialized?

That's just one of the possible problems. The father the MTRRs we run with
diverged from what firmware puts in place, the bigger the possible trouble.
I think the restoring better is done as being switched to here again. The
absence of any discussion of MTRRs in that earlier change leaves me pretty
certain that the behavioral change there wasn't intended. Andrew is usually
pretty good at spelling out all intended effects.

Jan