Re: [Intel-gfx] [cache coherency bug] i915 and PAT attributes

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Dec 16, 2022 at 03:30:13PM +0000, Andrew Cooper wrote:
> On 08/12/2022 1:55 pm, Marek Marczykowski-Górecki wrote:
> > Hi,
> >
> > There is an issue with i915 on Xen PV (dom0). The end result is a lot of
> > glitches, like here: https://openqa.qubes-os.org/tests/54748#step/startup/8
> > (this one is on ADL, Linux 6.1-rc7 as a Xen PV dom0). It's using Xorg
> > with "modesetting" driver.
> >
> > After some iterations of debugging, we narrowed it down to i915 handling
> > caching. The main difference is that PAT is setup differently on Xen PV
> > than on native Linux. Normally, Linux does have appropriate abstraction
> > for that, but apparently something related to i915 doesn't play well
> > with it. The specific difference is:
> > native linux:
> > x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- WT
> > xen pv:
> > x86/PAT: Configuration [0-7]: WB  WT  UC- UC  WC  WP  UC  UC
> >                                   ~~          ~~      ~~  ~~
> >
> > The specific impact depends on kernel version and the hardware. The most
> > severe issues I see on >=ADL, but some older hardware is affected too -
> > sometimes only if composition is disabled in the window manager.
> > Some more information is collected at
> > https://github.com/QubesOS/qubes-issues/issues/4782 (and few linked
> > duplicates...).
> >
> > Kind-of related commit is here:
> > https://github.com/torvalds/linux/commit/bdd8b6c98239cad ("drm/i915:
> > replace X86_FEATURE_PAT with pat_enabled()") - it is the place where
> > i915 explicitly checks for PAT support, so I'm cc-ing people mentioned
> > there too.
> >
> > Any ideas?
> >
> > The issue can be easily reproduced without Xen too, by adjusting PAT in
> > Linux:
> > -----8<-----
> > diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> > index 66a209f7eb86..319ab60c8d8c 100644
> > --- a/arch/x86/mm/pat/memtype.c
> > +++ b/arch/x86/mm/pat/memtype.c
> > @@ -400,8 +400,8 @@ void pat_init(void)
> >  		 * The reserved slots are unused, but mapped to their
> >  		 * corresponding types in the presence of PAT errata.
> >  		 */
> > -		pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
> > -		      PAT(4, WB) | PAT(5, WP) | PAT(6, UC_MINUS) | PAT(7, WT);
> > +		pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) |
> > +		      PAT(4, WC) | PAT(5, WP) | PAT(6, UC)       | PAT(7, UC);
> >  	}
> >  
> >  	if (!pat_bp_initialized) {
> > -----8<-----
> >
> 
> Hello, can anyone help please?
> 
> Intel's CI has taken this reproducer of the bug, and confirmed the
> regression. 
> https://lore.kernel.org/intel-gfx/Y5Hst0bCxQDTN7lK@mail-itl/T/#m4480c15a0d117dce6210562eb542875e757647fb
> 
> We're reasonably confident that it is an i915 bug (given the repro with
> no Xen in the mix), but we're out of any further ideas.

I don't think we have any code that assumes anything about the PAT,
apart from WC being available (which seems like it should still be
the case with your modified PAT). I suppose you'll just have to 
start digging from pgprot_writecombine()/noncached() and make sure
everything ends up using the correct PAT entry.

-- 
Ville Syrjälä
Intel