Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR

[Chuck Zmudzinski]

On 7/15/2022 3:53 PM, Chuck Zmudzinski wrote:
> On 7/15/2022 6:05 AM, Jan Beulich wrote:
> > On 15.07.2022 04:07, Chuck Zmudzinski wrote:
> > > On 7/14/2022 1:30 AM, Thorsten Leemhuis wrote:
> > >> On 13.07.22 03:36, Chuck Zmudzinski wrote:
> > >>> The commit 99c13b8c8896d7bcb92753bf
> > >>> ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> > >>> incorrectly failed to account for the case in init_cache_modes() when
> > >>> CPUs do support PAT and falsely reported PAT to be disabled when in
> > >>> fact PAT is enabled. In some environments, notably in Xen PV domains,
> > >>> MTRR is disabled but PAT is still enabled, and that is the case
> > >>> that the aforementioned commit failed to account for.
> > >>>
> > >>> As an unfortunate consequnce, the pat_enabled() function currently does
> > >>> not correctly report that PAT is enabled in such environments. The fix
> > >>> is implemented in init_cache_modes() by setting pat_bp_enabled to true
> > >>> in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf
> > >>> ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed
> > >>> to account for.
> > >>>
> > >>> This approach arranges for pat_enabled() to return true in the Xen PV
> > >>> environment without undermining the rest of PAT MSR management logic
> > >>> that considers PAT to be disabled: Specifically, no writes to the PAT
> > >>> MSR should occur.
> > >>>
> > >>> This patch fixes a regression that some users are experiencing with
> > >>> Linux as a Xen Dom0 driving particular Intel graphics devices by
> > >>> correctly reporting to the Intel i915 driver that PAT is enabled where
> > >>> previously it was falsely reporting that PAT is disabled. Some users
> > >>> are experiencing system hangs in Xen PV Dom0 and all users on Xen PV
> > >>> Dom0 are experiencing reduced graphics performance because the keying of
> > >>> the use of WC mappings to pat_enabled() (see arch_can_pci_mmap_wc())
> > >>> means that in particular graphics frame buffer accesses are quite a bit
> > >>> less performant than possible without this patch.
> > >>>
> > >>> Also, with the current code, in the Xen PV environment, PAT will not be
> > >>> disabled if the administrator sets the "nopat" boot option. Introduce
> > >>> a new boolean variable, pat_force_disable, to forcibly disable PAT
> > >>> when the administrator sets the "nopat" option to override the default
> > >>> behavior of using the PAT configuration that Xen has provided.
> > >>>
> > >>> For the new boolean to live in .init.data, init_cache_modes() also needs
> > >>> moving to .init.text (where it could/should have lived already before).
> > >>>
> > >>> Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> > >>
> > >> BTW, "submitting-patches.rst" says it should just be "the first 12
> > >> characters of the SHA-1 ID"
> > > 
> > > Actually it says "at least", so more that 12 is It is probably safest
> > > to put the entire SHA-1 ID in because of the possibility of
> > > a collision. At least that's how I understand what
> > > submitting-patches.rst.
> > > 
> > >>
> > >>> Co-developed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > >>> Cc: stable@xxxxxxxxxxxxxxx
> > >>> Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
> > >>
> > >> Sorry, have to ask: is this supposed to finally fix this regression?
> > >> https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/
> > > 
> > > Yes that's the first report I saw to lkml about this isssue. So if I submit
> > > a v3 I will include that. But my patch does not have a sign-off from the
> > > Co-developer as I mentioned in a message earlier today, and the
> > > discussion that has ensued leads me to think a better solution is to just
> > > revert the commit in the i915 driver instead, and leave the bigger questions
> > > for Juergen Gross and his plans to re-work the x86/PAT code in September,
> > > as he said on this thread in the last couple of days. This patch is dead
> > > now,
> > > as far as I can tell, because the Co-developer is not cooperating.
> >
> > ???
> >
> > Jan
>
> I think I recall you said my patch proves I don't want your S-o-b. I
> also want
> to add some useful logs with your approach, probably a pr_warn, which you
> are not interested in doing. I think it is probably necessary, under the
> current
> situation, to warn all users of Xen/Linux that Linux on Xen is not
> guaranteed
> to be secure and full-featured anymore. That is also why I think the Linux
> maintainers are ignoring your patch. They are basically saying Xen is just
> some weird one-off thing, I can't remember who said it, but I did read
> it here
> in some of the comments on your patch, and you do not seem to be listening
> to and responding to what the Linux developers are asking you to do.
>
> Two things I see here in my efforts to get a patch to fix this regression:
>
> 1. Does Xen have plans to give Linux running in Dom0 write-access to the
> PAT MSR?
>
> 2. Does Xen have plans to expose MTRRs to Linux running in Dom0?
>
> These don't have to be the defaults for Dom0. But can Xen at least make
> these as supported configurations for Linux? Then, the problem of Xen
> being some weird one-off thing goes away. At least that's how I see
> the situation. Maybe Xen can provide these for Linux in Dom0, but
> from what I've read here, it seems Xen is not willing to do that for
> Linux. Do I understand that correctly?
>
> Chuck

???

Chuck