linux-kernel - [PATCH] x86/pat: fix pat_x_mtrr

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Message-Id: <20230110065427.20767-1-jgross@suse.com>
Date:   Tue, 10 Jan 2023 07:54:27 +0100
From:   Juergen Gross <jgross@...e.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     mikelley@...rosoft.com, Juergen Gross <jgross@...e.com>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/pat: fix pat_x_mtrr_type() for MTRR disabled case

Since commit 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly
reflect state when running on Xen") PAT can be enabled without MTRR.

This has resulted in problems e.g. for a SEV-SNP guest running under
Hyper-V, when trying to establish a new mapping via memremap() with
WB caching mode, as pat_x_mtrr_type() will call mtrr_type_lookup(),
which in turn is returning MTRR_TYPE_INVALID due to MTRR being
disabled in this configuration. The result is a mapping with UC-
caching, leading to severe performance degradation.

Fix that by handling MTRR_TYPE_INVALID the same way as MTRR_TYPE_WRBACK
in pat_x_mtrr_type().

Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")
Reported-by: Michael Kelley (LINUX) <mikelley@...rosoft.com>
Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/mm/pat/memtype.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 46de9cf5c91d..fb4b1b5e0dea 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -387,7 +387,8 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end,
 		u8 mtrr_type, uniform;
 
 		mtrr_type = mtrr_type_lookup(start, end, &uniform);
-		if (mtrr_type != MTRR_TYPE_WRBACK)
+		if (mtrr_type != MTRR_TYPE_WRBACK &&
+		    mtrr_type != MTRR_TYPE_INVALID)
 			return _PAGE_CACHE_MODE_UC_MINUS;
 
 		return _PAGE_CACHE_MODE_WB;
-- 
2.35.3