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

[Chuck Zmudzinski <brchuckz@netscape.net>]

On 7/13/2022 9:52 AM, Jan Beulich wrote:
> 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

What conclusion do you draw from these facts?

1. Linus' regression rule is a high priority in Linux
2. Security concerns are even a higher priority in Linux
3. You and I have been trying to fix a regression for the past two months
4. The ones who can fix the regression have not accepted our patches.
5. I have been asked to help backport my fix to all stable branches.

Chuck