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 MSR is not supported
due to not having loaded the appropriate microcode.

Check for X86_FEATURE_SRBDS_CTRL which is set only when the respective
microcode which adds MSR_IA32_MCU_OPT_CTRL is loaded.

Based on a patch by Thadeu Lima de Souza Cascardo <cascardo@canonical.com>.

  [ bp: Massage commit message. ]
Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220401074517.1848264-1-ricardo.canuelo@collabora.com

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) {