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

[Jan Beulich <jbeulich@suse.com>]

On 13.07.2022 15:49, Chuck Zmudzinski wrote:
> On 7/13/2022 9:34 AM, Jan Beulich wrote:
>> 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.
>>
>> Jan
> 
> IOW, you are saying PAT has been broken on Xen for a
> long time, and it is necessary to fix it now not only on
> master, but also on all the stable branches.
> 
> Why is it necessary to do it on all the stable branches?

I'm not saying that's _necessary_ (but I think it would make sense),
and I'm not the one to decide whether or how far to backport this.

Jan