linux-kernel - [PATCH] x86/speculation/srbds: do not try to turn mitigation off when not supported

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Message-Id: <20200609174313.2600320-1-cascardo@canonical.com>
Date:   Tue,  9 Jun 2020 14:43:13 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     linux-kernel@...r.kernel.org
Cc:     Mark Gross <mgross@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        John Johansen <john.johansen@...onical.com>,
        Steve Beattie <sbeattie@...ntu.com>
Subject: [PATCH] x86/speculation/srbds: do not try to turn mitigation off when not supported

When SRBDS is mitigated by TSX OFF, update_srbds_msr will still read and
write to MSR_IA32_MCU_OPT_CTRL even when that is not supported by the
microcode.

Checking for X86_FEATURE_SRBDS_CTRL as a CPU feature available makes more
sense than checking for SRBDS_MITIGATION_UCODE_NEEDED as the found
"mitigation".

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Acked-by: John Johansen <john.johansen@...onical.com>
Acked-by: Steve Beattie <sbeattie@...ntu.com>
Cc: stable@...r.kernel.org
---
 arch/x86/kernel/cpu/bugs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b6f887be440c..ee5bdca7fd30 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -432,7 +432,7 @@ void update_srbds_msr(void)
 	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		return;
 
-	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
+	if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
 		return;
 
 	rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);
-- 
2.25.1