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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode availability
On 25.05.22 09:45, 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.cI 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 driverThe 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? No, doesn't seem so. I'm just reviewing the suggested fix: https://lore.kernel.org/lkml/Yo0LwmVUDSBZb44K@itl-email/ Juergen Attachment:
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