linux-kernel - [PATCH v3] x86/speculation/srbds: do not try to turn mitigation off when not supported

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Date:   Fri,  1 Apr 2022 09:45:17 +0200
From:   Ricardo Cañuelo <ricardo.canuelo@...labora.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ricardo Cañuelo <ricardo.canuelo@...labora.com>,
        Borislav Petkov <bp@...en8.de>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Mark Gross <mgross@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Johansen <john.johansen@...onical.com>,
        Steve Beattie <sbeattie@...ntu.com>, kernel@...labora.com
Subject: [PATCH v3] 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>
Signed-off-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@...labora.com>
Tested-by: Ricardo Cañuelo <ricardo.canuelo@...labora.com>
---

Changes in v3:
  - Simplified and more explicit solution proposed by Boris

 arch/x86/kernel/cpu/bugs.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6296e1ebed1d..d879a6c93609 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -446,6 +446,13 @@ void update_srbds_msr(void)
 	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
 		return;
 
+	/*
+	 * A MDS_NO CPU for which SRBDS mitigation is not needed due to TSX
+	 * being disabled and it hasn't received the SRBDS MSR microcode.
+	 */
+	if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
+		return;
+
 	rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);
 
 	switch (srbds_mitigation) {
-- 
2.25.1