linux-kernel - [PATCH] x86/cpu/intel: Skip CPA cache flush on CPUs with cache self-snooping

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Date:   Mon, 15 Jul 2019 16:21:09 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     tglx@...utronix.de, luto@...capital.net, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>
Subject: [PATCH] x86/cpu/intel: Skip CPA cache flush on CPUs with cache self-snooping

CPUs which have self-snooping capability can handle conflicting
memory type across CPUs by snooping its own cache. Commit #fd329f276ecaa
("x86/mtrr: Skip cache flushes on CPUs with cache self-snooping")
avoids cache flushes when MTRR registers are programmed. The Page
Attribute Table (PAT) is a companion feature to the MTRRs, and according
to section 11.12.4 of the Intel 64 and IA 32 Architectures Software
Developer's Manual, if the CPU supports cache self-snooping, it is not
necessary to flush caches when remapping a page that was previously
mapped as a different memory type.

Note that commit #1e03bff360010
("x86/cpu/intel: Clear cache self-snoop capability in CPUs with known errata")
cleared cache self-snoop capability for CPUs where conflicting memory types
lead to unpredictable behavior, machine check errors, or hangs.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 arch/x86/mm/pageattr.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 6a9a77a403c9..e2704996f9c5 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1725,10 +1725,11 @@ static int change_page_attr_set_clr(unsigned long *addr, int numpages,
 		goto out;

 	/*
-	 * No need to flush, when we did not set any of the caching
-	 * attributes:
+	 * No need to flush when CPU supports self-snoop or
+	 * when we did not set any of the caching attributes:
 	 */
-	cache = !!pgprot2cachemode(mask_set);
+	cache = !static_cpu_has(X86_FEATURE_SELFSNOOP) &&
+		pgprot2cachemode(mask_set);

 	/*
 	 * On error; flush everything to be sure.
-- 
2.21.0