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

[Juergen Gross <jgross@xxxxxxxx>]

On 13.07.22 15:34, 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.
There is Documentation/x86/pat.rst which rather clearly states, how
"nopat" is meant to work. It should not change the contents of the
PAT MSR and keep it just as it was set at boot time (the doc talks
about the "BIOS" setting of the MSR, and I guess in the Xen case
the hypervisor is kind of acting as the BIOS).

The question is, whether "nopat" needs to be translated to
pat_enabled() returning "false". I've found exactly one case where
"nopat" seems to be required for a driver to work correctly, but
this driver (drivers/media/pci/ivtv/ivtvfb.c) seems to rely on
MTRR availability, which isn't supported in Xen PV guests.

Other than that the "nopat" option seems to be a chicken bit from
the early days of PAT usage, which probably isn't really needed any
more. So I wouldn't be worried to drop "nopat" having any effect
on the system in Xen PV guests.

On bare metal it should still work, of course, if only for said
driver.

> > 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?
Note that it is not me to accept such a patch, this would be one
of the x86 maintainers (e.g. Boris).


Juergen

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