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

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
> ---
> 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.

> +               /*
> +                * 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

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