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

["Roger Pau Monné" <roger.pau@citrix.com>]

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?

Thanks, Roger.