[tip: x86/bugs] x86/srso: Don't probe microcode in a guest

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     3f267f825c98e3b3db0194645ac68ea154eac19d
Gitweb:        https://git.kernel.org/tip/3f267f825c98e3b3db0194645ac68ea154eac19d
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Fri, 25 Aug 2023 00:01:34 -07:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 25 Aug 2023 11:21:58 +02:00

x86/srso: Don't probe microcode in a guest

To support live migration, the hypervisor sets the "lowest common
denominator" of features.  Probing the microcode isn't allowed because
any detected features might go away after a migration.

As Andy Cooper states:

  "Linux must not probe microcode when virtualised.  What it may see
  instantaneously on boot (owing to MSR_PRED_CMD being fully passed
  through) is not accurate for the lifetime of the VM."

Rely on the hypervisor to set the needed IBPB_BRTYPE and SBPB bits.

Fixes: 1b5277c0ea0b ("x86/srso: Add SRSO_NO support")
Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Link: https://lore.kernel.org/r/3e293282e96b9fe1835c8bd22d1aaac07d9628e7.1692919072.git.jpoimboe@xxxxxxxxxx
---
 arch/x86/kernel/cpu/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index b08af92..28e77c5 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -767,7 +767,7 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	if (cpu_has(c, X86_FEATURE_TOPOEXT))
 		smp_num_siblings = ((cpuid_ebx(0x8000001e) >> 8) & 0xff) + 1;
 
-	if (!cpu_has(c, X86_FEATURE_IBPB_BRTYPE)) {
+	if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !cpu_has(c, X86_FEATURE_IBPB_BRTYPE)) {
 		if (c->x86 == 0x17 && boot_cpu_has(X86_FEATURE_AMD_IBPB))
 			setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
 		else if (c->x86 >= 0x19 && !wrmsrl_safe(MSR_IA32_PRED_CMD, PRED_CMD_SBPB)) {