Message-ID: <ec4b2791-886f-4d52-ab39-b0c07489c4ca@suse.com>
Date:   Thu, 14 Jul 2022 07:40:57 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Chuck Zmudzinski <brchuckz@....com>, linux-kernel@...r.kernel.org
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Jane Chu <jane.chu@...cle.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Jan Beulich <jbeulich@...e.com>,
        xen-devel@...ts.xenproject.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT
 without MTRR

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@...e.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Chuck Zmudzinski <brchuckz@....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.

> +		/*
> +		 * 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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