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

From: Jan Beulich
Date: Wed Jul 13 2022 - 09:34:30 EST

On 13.07.2022 13:10, Chuck Zmudzinski wrote:
> On 7/13/2022 6:36 AM, Chuck Zmudzinski wrote:
>> On 7/13/2022 5:09 AM, Jan Beulich wrote:
>>> On 13.07.2022 10:51, Chuck Zmudzinski wrote:
>>>> On 7/13/22 2:18 AM, Jan Beulich wrote:
>>>>> On 13.07.2022 03:36, Chuck Zmudzinski wrote:
>>>>>> v2: *Add force_pat_disabled variable to fix "nopat" on Xen PV (Jan Beulich)
>>>>>> *Add the necessary code to incorporate the "nopat" fix
>>>>>> *void init_cache_modes(void) -> void __init init_cache_modes(void)
>>>>>> *Add Jan Beulich as Co-developer (Jan has not signed off yet)
>>>>>> *Expand the commit message to include relevant parts of the commit
>>>>>> message of Jan Beulich's proposed patch for this problem
>>>>>> *Fix 'else if ... {' placement and indentation
>>>>>> *Remove indication the backport to stable branches is only back to 5.17.y
>>>>>> I think these changes address all the comments on the original patch
>>>>>> I added Jan Beulich as a Co-developer because Juergen Gross asked me to
>>>>>> include Jan's idea for fixing "nopat" that was missing from the first
>>>>>> version of the patch.
>>>>> You've sufficiently altered this change to clearly no longer want my
>>>>> S-o-b; unfortunately in fact I think you broke things:
>>>> Well, I hope we can come to an agreement so I have
>>>> your S-o-b. But that would probably require me to remove
>>>> Juergen's R-b.
>>>>>> @@ -292,7 +294,7 @@ void init_cache_modes(void)
>>>>>> rdmsrl(MSR_IA32_CR_PAT, pat);
>>>>>> }
>>>>>> - if (!pat) {
>>>>>> + if (!pat || pat_force_disabled) {
>>>>> By checking the new variable here ...
>>>>>> /*
>>>>>> * No PAT. Emulate the PAT table that corresponds to the two
>>>>>> * cache bits, PWT (Write Through) and PCD (Cache Disable).
>>>>>> @@ -313,6 +315,16 @@ void init_cache_modes(void)
>>>>>> */
>>>>>> pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) |
>>>>>> PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
>>>>> ... you put in place a software view which doesn't match hardware. I
>>>>> continue to think that ...
>>>>>> + } else if (!pat_bp_enabled) {
>>>>> ... the variable wants checking here instead (at which point, yes,
>>>>> this comes quite close to simply being a v2 of my original patch).
>>>>> By using !pat_bp_enabled here you actually broaden where the change
>>>>> would take effect. Iirc Boris had asked to narrow things (besides
>>>>> voicing opposition to this approach altogether). Even without that
>>>>> request I wonder whether you aren't going to far with this.
>>>>> Jan
>>>> I thought about checking for the administrator's "nopat"
>>>> setting where you suggest which would limit the effect
>>>> of "nopat" to not reporting PAT as enabled to device
>>>> drivers who query for PAT availability using pat_enabled().
>>>> The main reason I did not do that is that due to the fact
>>>> that we cannot write to the PAT MSR, we cannot really
>>>> disable PAT. But we come closer to respecting the wishes
>>>> of the administrator by configuring the caching modes as
>>>> if PAT is actually disabled by the hardware or firmware
>>>> when in fact it is not.
>>>> What would you propose logging as a message when
>>>> we report PAT as disabled via pat_enabled()? The main
>>>> reason I did not choose to check the new variable in the
>>>> new 'else if' block is that I could not figure out what to
>>>> tell the administrator in that case. I think we would have
>>>> to log something like, "nopat is set, but we cannot disable
>>>> PAT, doing our best to disable PAT by not reporting PAT
>>>> as enabled via pat_enabled(), but that does not guarantee
>>>> that kernel drivers and components cannot use PAT if they
>>>> query for PAT support using boot_cpu_has(X86_FEATURE_PAT)
>>>> instead of pat_enabled()." However, I acknowledge WC mappings
>>>> would still be disabled because arch_can_pci_mmap_wc() will
>>>> be false if pat_enabled() is false.
>>>> Perhaps we also need to log something if we keep the
>>>> check for "nopat" where I placed it. We could say something
>>>> like: "nopat is set, but we cannot disable hardware/firmware
>>>> PAT support, so we are emulating as if there is no PAT support
>>>> which puts in place a software view that does not match
>>>> hardware."
>>>> No matter what, because we cannot write to PAT MSR in
>>>> the Xen PV case, we probably need to log something to
>>>> explain the problems associated with trying to honor the
>>>> administrator's request. Also, what log level should it be.
>>>> Should it be a pr_warn instead of a pr_info?
>>> I'm afraid I'm the wrong one to answer logging questions. As you
>>> can see from my original patch, I didn't add any new logging (and
>>> no addition was requested in the comments that I have got). I also
>>> don't think "nopat" has ever meant "disable PAT", as the feature
>>> is either there or not. Instead I think it was always seen as
>>> "disable fiddling with PAT", which by implication means using
>>> whatever is there (if the feature / MSR itself is available).
>> IIRC, I do think I mentioned in the comments on your patch that
>> it would be preferable to mention in the commit message that
>> your patch would change the current behavior of "nopat" on
>> Xen. The question is, how much do we want to change the
>> current behavior of "nopat" on Xen. I think if we have to change
>> the current behavior of "nopat" on Xen and if we are going
>> to propagate that change to all current stable branches all
>> the way back to 4.9.y,, we better make a lot of noise about
>> what we are doing here.
>> Chuck
> And in addition, if we are going to backport this patch to
> all current stable branches, we better have a really, really,
> good reason for changing the behavior of "nopat" on Xen.
> Does such a reason exist?

Well, the simple reason is: It doesn't work the same way under Xen
and non-Xen (in turn because, before my patch or whatever equivalent
work, things don't work properly anyway, PAT-wise). Yet it definitely
ought to behave the same everywhere, imo.


> Or perhaps, Juergen, could you
> accept a v3 of my patch that does not need to decide
> how to backport the change to "nopat" to the stable branches
> that are affected by the current behavior of "nopat" on Xen?
> To do such a v3, I would just have to fix the style problems
> with my original patch and not come to an agreement with
> Jan about how to deal with the "nopat" problem.
> Chuck
> Chuck