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

[Chuck Zmudzinski <brchuckz@netscape.net>]

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@suse.com/
> ...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@netscape.net/
>>
>>
>> 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