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

[Chuck Zmudzinski <brchuckz@netscape.net>]

On 7/14/2022 10:19 PM, Chuck Zmudzinski wrote:
> On 7/14/2022 1:40 AM, Juergen Gross wrote:
> > On 13.07.22 03:36, Chuck Zmudzinski wrote:
> > > The commit 99c13b8c8896d7bcb92753bf
> > > ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> > > incorrectly failed to account for the case in init_cache_modes() when
> > > CPUs do support PAT and falsely reported PAT to be disabled when in
> > > fact PAT is enabled. In some environments, notably in Xen PV domains,
> > > MTRR is disabled but PAT is still enabled, and that is the case
> > > that the aforementioned commit failed to account for.
> > > 
> > > As an unfortunate consequnce, the pat_enabled() function currently does
> > > not correctly report that PAT is enabled in such environments. The fix
> > > is implemented in init_cache_modes() by setting pat_bp_enabled to true
> > > in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf
> > > ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed
> > > to account for.
> > > 
> > > This approach arranges for pat_enabled() to return true in the Xen PV
> > > environment without undermining the rest of PAT MSR management logic
> > > that considers PAT to be disabled: Specifically, no writes to the PAT
> > > MSR should occur.
> > > 
> > > This patch fixes a regression that some users are experiencing with
> > > Linux as a Xen Dom0 driving particular Intel graphics devices by
> > > correctly reporting to the Intel i915 driver that PAT is enabled where
> > > previously it was falsely reporting that PAT is disabled. Some users
> > > are experiencing system hangs in Xen PV Dom0 and all users on Xen PV
> > > Dom0 are experiencing reduced graphics performance because the keying of
> > > the use of WC mappings to pat_enabled() (see arch_can_pci_mmap_wc())
> > > means that in particular graphics frame buffer accesses are quite a bit
> > > less performant than possible without this patch.
> > > 
> > > Also, with the current code, in the Xen PV environment, PAT will not be
> > > disabled if the administrator sets the "nopat" boot option. Introduce
> > > a new boolean variable, pat_force_disable, to forcibly disable PAT
> > > when the administrator sets the "nopat" option to override the default
> > > behavior of using the PAT configuration that Xen has provided.
> > > 
> > > For the new boolean to live in .init.data, init_cache_modes() also needs
> > > moving to .init.text (where it could/should have lived already before).
> > > 
> > > Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> > > Co-developed-by: Jan Beulich <jbeulich@suse.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
> > > ---
> > > 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.
> > > 
> > > The patch has been tested, it works as expected with and without nopat
> > > in the Xen PV Dom0 environment. That is, "nopat" causes the system to
> > > exhibit the effects and problems that lack of PAT support causes.
> > > 
> > >   arch/x86/mm/pat/memtype.c | 16 ++++++++++++++--
> > >   1 file changed, 14 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> > > index d5ef64ddd35e..10a37d309d23 100644
> > > --- a/arch/x86/mm/pat/memtype.c
> > > +++ b/arch/x86/mm/pat/memtype.c
> > > @@ -62,6 +62,7 @@
> > >   
> > >   static bool __read_mostly pat_bp_initialized;
> > >   static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);
> > > +static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT);
> > >   static bool __read_mostly pat_bp_enabled;
> > >   static bool __read_mostly pat_cm_initialized;
> > >   
> > > @@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason)
> > >   static int __init nopat(char *str)
> > >   {
> > >   	pat_disable("PAT support disabled via boot option.");
> > > +	pat_force_disabled = true;
> > >   	return 0;
> > >   }
> > >   early_param("nopat", nopat);
> > > @@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat)
> > >   	wrmsrl(MSR_IA32_CR_PAT, pat);
> > >   }
> > >   
> > > -void init_cache_modes(void)
> > > +void __init init_cache_modes(void)
> > >   {
> > >   	u64 pat = 0;
> > >   
> > > @@ -292,7 +294,7 @@ void init_cache_modes(void)
> > >   		rdmsrl(MSR_IA32_CR_PAT, pat);
> > >   	}
> > >   
> > > -	if (!pat) {
> > > +	if (!pat || pat_force_disabled) {
> >
> > Can we just remove this modification and ...
> >
> > >   		/*
> > >   		 * 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);
> > > +	} else if (!pat_bp_enabled) {
> >
> > ... use
> >
> > +	} else if (!pat_bp_enabled && !pat_force_disabled) {
> >
> > here?
> >
> > This will result in the desired outcome in all cases IMO: If PAT wasn't
> > disabled via "nopat" and the PAT MSR has a non-zero value (from BIOS or
> > Hypervisor) and PAT has been disabled implicitly (e.g. due to lack of
> > MTRR), then PAT will be set to "enabled" again.
>
> With that, you can also completely remove the new Boolean - it
> will be a meaningless variable wasting memory. This will also make
> my patch more or less do what Jan's patch does - the "nopat" option
> will not cause the situation when the PAT MSR does not match the
> software view. So you are basically proposing just going back to
> my original patch, after fixing the style problems, of course. That
> also would solve the problem of needing Jan's S-o-b. I am inclined,
> however, to wait for a maintainer who has power to actually do the
> commit, to make a comment. Your R-b to my v2 did not have much clout
> with the actual maintainers, as far as I can tell. I am somewhat annoyed
> that it was at your suggestion that my v2 ended up confusing the
> main issue, the regression, with the red herring of the "nopat"
> option.
>
> Chuck

Actually, what your change does depend on keeping
pat_force_disable, but after all the discussion and
further thinking about this, I would prefer that you
give a R-b to v3 as simply my original patch with the
style fixed. I think it is wrong to confuse the regression
with the "nopat" issue. If you and Jan want to do a patch
for the "nopat" issue, that is your decision. I am not interested
in that. I am interested in fixing the regression. Also, I am
not included to formally submit v3 until Dave, Andy, Boris, or
someone else with more clout here on Linux expresses
interest in giving this idea an R-b.

Chuck

>
>
> > > +		/*
> > > +		 * In some environments, specifically Xen PV, PAT
> > > +		 * initialization is skipped because MTRRs are disabled even
> > > +		 * though PAT is available. In such environments, set PAT to
> > > +		 * enabled to correctly indicate to callers of pat_enabled()
> > > +		 * that CPU support for PAT is available.
> > > +		 */
> > > +		pat_bp_enabled = true;
> > > +		pr_info("x86/PAT: PAT enabled by init_cache_modes\n");
> > >   	}
> > >   
> > >   	__init_cache_modes(pat);
> >
> >
> > Juergen
>
>