Re: [RFC PATCH 8/8] Use Linux's PAT

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Dec 06, 2022 at 01:01:41PM -0500, Demi Marie Obenour wrote:
> On Tue, Dec 06, 2022 at 11:38:03AM +0000, Andrew Cooper wrote:
> > On 06/12/2022 04:33, Demi Marie Obenour wrote:
> > > This is purely for testing, to see if it works around a bug in i915.  It
> > > is not intended to be merged.
> > >
> > > NOT-signed-off-by: DO NOT MERGE
> > 
> > Following up on Marek's report on IRC/Matrix, you're saying that this
> > change does actually fix screen corruption issues on AlderLake, and
> > something on TigerLake too?
> 
> Correct
> 
> > If that is actually the case, then one of two things is happening.  Either,
> > 
> > 1) Drivers in Linux are bypassing the regular caching APIs, or
> 
> This would not surprise me at all.
> 
> > 2) The translation logic between Linux's idea of cacheability and Xen's
> > PAT values is buggy.
> 
> How could I check for this?

See Andy's unit test idea on #xendevel:

    as a pretty simple "unit" test in dom0, it might be a good idea to
    have a module which watches the PTE in question, and cycles through
    various of the memremap_*() APIs and checks the raw PTE that gets
    written after Linux and Xen are done fighting with it

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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