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From:   Chuck Zmudzinski <brchuckz@aol.com>
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        xen-devel@lists.xenproject.org, stable@vger.kernel.org
Subject: [PATCH] x86/PAT: Report PAT on CPUs that support PAT without MTRR
Date:   Tue, 12 Jul 2022 14:20:37 -0400
Message-Id: <2885cdcaccffd287ef69c7509056ddf183a38a0e.1657647656.git.brchuckz@aol.com>
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References: <2885cdcaccffd287ef69c7509056ddf183a38a0e.1657647656.git.brchuckz.ref@aol.com>
Precedence: bulk

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 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.

Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
---
Reminder: This patch is a regression fix that is needed on stable
versions 5.17 and later.

 arch/x86/mm/pat/memtype.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index d5ef64ddd35e..0f2417bd1b40 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -315,6 +315,18 @@ void init_cache_modes(void)
 		      PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
 	}
 
+	else if (!pat_bp_enabled) {
+	/*
+	 * 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);
 }
 
-- 
2.36.1