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

[Chuck Zmudzinski <brchuckz@netscape.net>]

On 7/15/2022 12:22 AM, Juergen Gross wrote:
> On 15.07.22 04:19, 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
>
> No, it is making a difference with "nopat" having been specified.
>
> In the Xen PV case we will have pat_bp_enabled == false due to the
> lack of MTRR. We don't want to set it to true if "nopat" has been
> specified on the command line, so pat_force_disabled should not be
> true when we are setting pat_bp_enabled to true again.
>
> > 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.
>
> I'm sorry for that.

I accept your apology. A few days back you indicated Boris was willing to
go along with the approach I was suggesting. Do you have any idea why
he didn't give me an R-b for either my original patch or v2 or at least tell
me what I would have to do to get his R-b?

Chuck

>
>
> Juergen