linux-kernel - [PATCH] x86/speculation/mds: Avoid clearing CPU buffers in native machine with old microcode

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Date:   Sun, 23 Jun 2019 11:35:03 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...nel.org, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, jgross@...e.com, ndesaulniers@...gle.com,
        gregkh@...uxfoundation.org,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>
Subject: [PATCH] x86/speculation/mds: Avoid clearing CPU buffers in native machine with old microcode

Commit 22dd8365088b ("x86/speculation/mds: Add mitigation mode VMWERV") add
an internal mitigation mode VWWERV which enables the invocation of the CPU
buffer clearing even if X86_FEATURE_MD_CLEAR is not set.

This wastes a few CPU cycles for native machine with an old microcode
unnecessorily. Avoid it by checking if it's running in native machine.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
---
 arch/x86/kernel/cpu/bugs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 03b4cc0..03f5a77 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -233,7 +233,9 @@ static void x86_amd_ssb_disable(void)
 
 static void __init mds_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off() ||
+	    (hypervisor_is_type(X86_HYPER_NATIVE) &&
+	    !boot_cpu_has(X86_FEATURE_MD_CLEAR))) {
 		mds_mitigation = MDS_MITIGATION_OFF;
 		return;
 	}
-- 
1.8.3.1