[Intel-gfx] [cache coherency bug] [hw bug?] i915 and PAT attributes
Marek Marczykowski-Górecki
marmarek at invisiblethingslab.com
Mon Jan 2 01:58:23 UTC 2023
On Sun, Jan 01, 2023 at 08:48:13PM -0500, Demi Marie Obenour wrote:
> On Sun, Jan 01, 2023 at 08:17:52PM -0500, Demi Marie Obenour wrote:
> > On Mon, Jan 02, 2023 at 02:00:51AM +0100, Marek Marczykowski-Górecki wrote:
> > > On Sun, Jan 01, 2023 at 07:03:18PM -0500, Demi Marie Obenour wrote:
> > > > On Mon, Jan 02, 2023 at 12:24:54AM +0100, Marek Marczykowski-Górecki wrote:
> > > > > On Thu, Dec 22, 2022 at 10:29:57AM +0200, Ville Syrjälä wrote:
> > > > > > 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.
> > > > >
> > > > > I tried several approach to this, without success. Here is an update on
> > > > > debugging (reported also on #intel-gfx live):
> > > > >
> > > > > I did several tests with different PAT configuration (by modifying Xen
> > > > > that sets the MSR). Full table is at https://pad.itl.space/sheet/#/2/sheet/view/HD1qT2Zf44Ha36TJ3wj2YL+PchsTidyNTFepW5++ZKM/
> > > > > Some highlights:
> > > > > - 1=WC, 4=WT - good
> > > > > - 1=WT, 4=WC - bad
> > > > > - 1=WT, 3=WC (4=WC too) - good
> > > > > - 1=WT, 5=WC - good
> > > > >
> > > > > So, for me it seems WC at index 4 is problematic for some reason.
> > > > >
> > > > > Next, I tried to trap all the places in arch/x86/xen/mmu_pv.c that
> > > > > write PTEs and verify requested cache attributes. There, it seems all
> > > > > the requested WC are properly translated (using either index 1, 3, 4, or
> > > > > 5 according to PAT settings). And then after reading PTE back, it indeed
> > > > > seems to be correctly set. I didn't added reading back after
> > > > > HYPERVISOR_update_va_mapping, but verified it isn't used for setting WC.
> > > > >
> > > > > Using the same method, I also checked that indexes that aren't supposed
> > > > > to be used (for example index 4 when both 3 and 4 are WC) indeed are not
> > > > > used. So, the hypothesis that specific indexes are hardcoded somewhere
> > > > > is unlikely.
> > > > >
> > > > > This all looks very weird to me. Any ideas?
> > > >
> > > > Old CPUs have had hardware errata that caused the top bit of the PAT
> > > > entry to be ignored in certain cases. Could modern CPUs be ignoring
> > > > this bit when accessing iGPU memory or registers? With WC at position
> > > > 4, this would cause WC to be treated as WB, which is consistent with the
> > > > observed behavior. WC at position 3 would not be impacted, and WC at
> > > > position 5 would be treated as WT which I expect to be safe. One way to
> > > > test this is to test 1=WB, 5=WC. If my hypothesis is correct, this
> > > > should trigger the bug, even if entry 1 in the PAT is unused because
> > > > entry 0 is also WB.
> > >
> > > This looks like a very probable situation, indeed 1=WB, 5=WC does
> > > trigger the bug! Specifically this layout:
> > >
> > > WB WB UC- UC WP WC WT UC
> >
> > What about WB WT WB UC WB WP WC UC- and WB WT WT UC WB WP WC UC-? Those
> > only differ in entry 2, which will not be used as it duplicates entry 0
> > or 1. Therefore, architecturally, these should behave identically. If
> > I am correct, the second will work fine, but the first will trigger the
> > bug.
Bingo! This also behaves as predicted.
So, it indeed looks like the _PAGE_PAT bit is ignored by the hardware,
even though set in relevant PTEs.
> Also worth testing:
>
> WB UC- UC WB WB WP WT WC
> WB UC- UC UC WB WP WT WC
>
> These differ only in (unused) entry 3.
I'll skip this, as I think it's pretty clear what will be the result.
But if somebody else think it's worth testing anyway, let me know.
--
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
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