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, 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. > 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? Ciao, Thorsten >>> The i915 driver relies on the memory subsytem >>> to provide it with an accurate test for the existence of >>> X86_FEATURE_PAT. I think your patch provides that more accurate >>> test so that bdd8b6c98239 could be re-applied when your patch is >>> committed. Juergen's patch would have to touch bdd8b6c98239 >>> with new functions that probably have unknown and unintended >>> consequences, so I think your approach is also better in that regard. >>> As regards your patch, there is just a disagreement about how the >>> i915 driver should behave if nopat is set. I agree the i915 driver >>> could do a better job handling that case, at least with better error >>> logs. >>> >>> Chuck >>> >>>>> I respond by saying a well-written driver should refuse to honor >>>>> the incorrect configuration requested by the user and instead >>>>> warn the user that it did not honor the incorrect kernel option. >>>>> >>>>> I am only presuming what your patch would do on my box based >>>>> on what I learned about this problem from my debugging. I can >>>>> also test your patch on my box to verify that my understanding of >>>>> it is correct. >>>>> >>>>> I also have not yet verified Juergen's patch will not fix it, but >>>>> I am almost certain it will not unless it is expanded so it also >>>>> touches i915_gem_object_pin_map() with the fix. I plan to test >>>>> his patch, but expanded so it touches that function also. >>>>> >>>>> I also plan to test your patch with and without nopat and report the >>>>> results in the thread where you posted your patch. Hopefully >>>>> by tomorrow I will have the results. >>>>> >>>>> Chuck > >
