Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Chuck Zmudzinski]

On 5/25/2022 3:45 AM, Thorsten Leemhuis wrote:
> On 24.05.22 20:32, Chuck Zmudzinski wrote:
> > On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
> > > On 20.05.22 16:48, Chuck Zmudzinski wrote:
> > > > On 5/20/2022 10:06 AM, Jan Beulich wrote:
> > > > > On 20.05.2022 15:33, Chuck Zmudzinski wrote:
> > > > > > On 5/20/2022 5:41 AM, Jan Beulich wrote:
> > > > > > > On 20.05.2022 10:30, Chuck Zmudzinski wrote:
> > > > > > > > On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
> > > > > > > > > On 5/20/2022 2:05 AM, Jan Beulich wrote:
> > > > > > > > > > On 20.05.2022 06:43, Chuck Zmudzinski wrote:
> > > > > > > > > > > On 5/4/22 5:14 AM, Juergen Gross wrote:
> > > > > > > > > > > > On 04.05.22 10:31, Jan Beulich wrote:
> > > > > > > > > > > > > On 03.05.2022 15:22, Juergen Gross wrote:
> > > > > > > > > > > > >
> > > > > > > > > > > > > ... these uses there are several more. You say nothing on why
> > > > > > > > > > > > > those want
> > > > > > > > > > > > > leaving unaltered. When preparing my earlier patch I did
> > > > > > > > > > > > > inspect them
> > > > > > > > > > > > > and came to the conclusion that these all would also better
> > > > > > > > > > > > > observe the
> > > > > > > > > > > > > adjusted behavior (or else I couldn't have left pat_enabled()
> > > > > > > > > > > > > as the
> > > > > > > > > > > > > only predicate). In fact, as said in the description of my
> > > > > > > > > > > > > earlier
> > > > > > > > > > > > > patch, in
> > > > > > > > > > > > > my debugging I did find the use in i915_gem_object_pin_map()
> > > > > > > > > > > > > to be
> > > > > > > > > > > > > the
> > > > > > > > > > > > > problematic one, which you leave alone.
> > > > > > > > > > > > Oh, I missed that one, sorry.
> > > > > > > > > > > That is why your patch would not fix my Haswell unless
> > > > > > > > > > > it also touches i915_gem_object_pin_map() in
> > > > > > > > > > > drivers/gpu/drm/i915/gem/i915_gem_pages.c
> > > > > > > > > > >
> > > > > > > > > > > > I wanted to be rather defensive in my changes, but I agree at
> > > > > > > > > > > > least
> > > > > > > > > > > > the
> > > > > > > > > > > > case in arch_phys_wc_add() might want to be changed, too.
> > > > > > > > > > > I think your approach needs to be more aggressive so it will fix
> > > > > > > > > > > all the known false negatives introduced by bdd8b6c98239
> > > > > > > > > > > such as the one in i915_gem_object_pin_map().
> > > > > > > > > > >
> > > > > > > > > > > I looked at Jan's approach and I think it would fix the issue
> > > > > > > > > > > with my Haswell as long as I don't use the nopat option. I
> > > > > > > > > > > really don't have a strong opinion on that question, but I
> > > > > > > > > > > think the nopat option as a Linux kernel option, as opposed
> > > > > > > > > > > to a hypervisor option, should only affect the kernel, and
> > > > > > > > > > > if the hypervisor provides the pat feature, then the kernel
> > > > > > > > > > > should not override that,
> > > > > > > > > > Hmm, why would the kernel not be allowed to override that? Such
> > > > > > > > > > an override would affect only the single domain where the
> > > > > > > > > > kernel runs; other domains could take their own decisions.
> > > > > > > > > >
> > > > > > > > > > Also, for the sake of completeness: "nopat" used when running on
> > > > > > > > > > bare metal has the same bad effect on system boot, so there
> > > > > > > > > > pretty clearly is an error cleanup issue in the i915 driver. But
> > > > > > > > > > that's orthogonal, and I expect the maintainers may not even care
> > > > > > > > > > (but tell us "don't do that then").
> > > > > > > > Actually I just did a test with the last official Debian kernel
> > > > > > > > build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
> > > > > > > > applied. In fact, the nopat option does *not* break the i915 driver
> > > > > > > > in 5.16. That is, with the nopat option, the i915 driver loads
> > > > > > > > normally on both the bare metal and on the Xen hypervisor.
> > > > > > > > That means your presumption (and the presumption of
> > > > > > > > the author of bdd8b6c98239) that the "nopat" option was
> > > > > > > > being observed by the i915 driver is incorrect. Setting "nopat"
> > > > > > > > had no effect on my system with Linux 5.16. So after doing these
> > > > > > > > tests, I am against the aggressive approach of breaking the i915
> > > > > > > > driver with the "nopat" option because prior to bdd8b6c98239,
> > > > > > > > nopat did not break the i915 driver. Why break it now?
> > > > > > > Because that's, in my understanding, is the purpose of "nopat"
> > > > > > > (not breaking the driver of course - that's a driver bug -, but
> > > > > > > having an effect on the driver).
> > > > > > I wouldn't call it a driver bug, but an incorrect configuration of the
> > > > > > kernel by the user.  I presume X86_FEATURE_PAT is required by the
> > > > > > i915 driver
> > > > > The driver ought to work fine without PAT (and hence without being
> > > > > able to make WC mappings). It would use UC instead and be slow, but
> > > > > it ought to work.
> > > > >
> > > > > > and therefore the driver should refuse to disable
> > > > > > it if the user requests to disable it and instead warn the user that
> > > > > > the driver did not disable the feature, contrary to what the user
> > > > > > requested with the nopat option.
> > > > > >
> > > > > > In any case, my test did not verify that when nopat is set in Linux
> > > > > > 5.16,
> > > > > > the thread takes the same code path as when nopat is not set,
> > > > > > so I am not totally sure that the reason nopat does not break the
> > > > > > i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
> > > > > > returns true even when nopat is set. I could test it with a custom
> > > > > > log message in 5.16 if that is necessary.
> > > > > >
> > > > > > Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
> > > > > > to return true in 5.16 when the user requests nopat?
> > > > > No, I'm not saying that. It was wrong for this construct to be used
> > > > > in the driver, which was fixed for 5.17 (and which had caused the
> > > > > regression I did observe, leading to the patch as a hopefully least
> > > > > bad option).
> > > > >
> > > > > > I think that is
> > > > > > just permitting a bad configuration to break the driver that a
> > > > > > well-written operating system should not allow. The i915 driver
> > > > > > was, in my opinion, correctly ignoring the nopat option in 5.16
> > > > > > because that option is not compatible with the hardware the
> > > > > > i915 driver is trying to initialize and setup at boot time. At least
> > > > > > that is my understanding now, but I will need to test it on 5.16
> > > > > > to be sure I understand it correctly.
> > > > > >
> > > > > > Also, AFAICT, your patch would break the driver when the nopat
> > > > > > option is set and only fix the regression introduced by bdd8b6c98239
> > > > > > when nopat is not set on my box, so your patch would
> > > > > > introduce a regression relative to Linux 5.16 and earlier for the
> > > > > > case when nopat is set on my box. I think your point would
> > > > > > be that it is not a regression if it is an incorrect user
> > > > > > configuration.
> > > > > Again no - my view is that there's a separate, pre-existing issue
> > > > > in the driver which was uncovered by the change. This may be a
> > > > > perceived regression, but is imo different from a real one.
> > > Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
> > > comes to the "no regressions rule". Just took a quick look at quotes
> > > from Linus
> > > https://www.kernel.org/doc/html/latest/process/handling-regressions.html
> > > and found this statement from Linus to back this up:
> > >
> > > ```
> > > One _particularly_ last-minute revert is the top-most commit (ignoring
> > > the version change itself) done just before the release, and while
> > > it's very annoying, it's perhaps also instructive.
> > >
> > > What's instructive about it is that I reverted a commit that wasn't
> > > actually buggy. In fact, it was doing exactly what it set out to do,
> > > and did it very well. In fact it did it _so_ well that the much
> > > improved IO patterns it caused then ended up revealing a user-visible
> > > regression due to a real bug in a completely unrelated area.
> > > ```
> > >
> > > He said that here:
> > > https://www.kernel.org/doc/html/latest/process/handling-regressions.html
> > >
> > > The situation is of course different here, but similar enough.
> > >
> > > > Since it is a regression, I think for now bdd8b6c98239 should
> > > > be reverted and the fix backported to Linux 5.17 stable until
> > > > the underlying memory subsystem can provide the i915 driver
> > > > with an updated test for the PAT feature that also meets the
> > > > requirements of the author of bdd8b6c98239 without breaking
> > > > the i915 driver.
> > > I'm not a developer and I'm don't known the details of this thread and
> > > the backstory of the regression, but it sounds like that's the approach
> > > that is needed here until someone comes up with a fix for the regression
> > > exposed by bdd8b6c98239.
> > >
> > > But if I'm wrong, please tell me.
> > You are mostly right, I think. Reverting bdd8b6c98239 fixes
> > it. There is another way to fix it, though.
> Yeah, I'm aware of it. But it seems...
>
> > The patch proposed
> > by Jan Beulich also fixes the regression on my system, so as
> > the person reporting this is a regression, I would also be satisfied
> > with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
> > posted his proposed patch here:
> >
> > https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@xxxxxxxx/
> ...that approach is not making any progress either?

Jan's approach does fix it on my system. There was some debate
about what the kernel nopat option should do, though. I don't
have a strong opinion on that and would accept Jan's patch
as a fix.

> Jan, can could provide a short status update here? I'd really like to
> get this regression fixed one way or another rather sooner than later,
> as this is taken way to long already IMHO.

I hope something is done soon also.

> > The only reservation I have about Jan's patch is that the commit
> > message does not clearly explain how the patch changes what
> > the nopat kernel boot option does. It doesn't affect me because
> > I don't use nopat, but it should probably be mentioned in the
> > commit message, as pointed out here:
> >
> > https://lore.kernel.org/lkml/bd9ed2c2-1337-27bb-c9da-dfc7b31d492c@xxxxxxxxxxxx/
> >
> >
> > Whatever fix for the regression exposed by bdd8b6c98239 also
> > needs to be backported to the stable versions 5.17 and 5.18.
> Sure.
>
> BTW, as you seem to be familiar with the issue: there is another report
> about a regression WRT to Xen and i915 (that is also not making really
> progress):
> https://lore.kernel.org/lkml/Yn%2FTgj1Ehs%2FBdpHp@itl-email/
>
> It's just a wild guess, but bould this somehow be related?

It could be, but I do not run a GUI in my Xen Dom0, so I have not
seen that issue.

Best regards,

Chuck