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

["Ricardo Cañuelo" <ricardo.canuelo@collabora.com>]

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@canonical.com>
Signed-off-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Tested-by: Ricardo Cañuelo <ricardo.canuelo@collabora.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