i915 and PAT attributes on Xen PV

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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<-----

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

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