[PATCH v2 1/3] cpu/bugs: Allow forcing Automatic IBRS with SNP enabled using spectre_v2=eibrs

[Kim Phillips <kim.phillips@amd.com>]

To allow this, do the SNP check in spectre_v2_select_mitigation()
processing instead of the original commit's implementation in
cpu_set_bug_bits().

Since SPECTRE_V2_CMD_AUTO logic falls through to SPECTRE_V2_CMD_FORCE,
double-check if SPECTRE_V2_CMD_FORCE is used before allowing
SPECTRE_V2_EIBRS with SNP enabled.

Also mute SPECTRE_V2_IBRS_PERF_MSG if SNP is enabled on an AutoIBRS
capable machine, since, in that case, the message doesn't apply.

Fixes: acaa4b5c4c85 ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@kernel.org
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
v2:
 - Address Dave Hansen's comment to adhere to using the IBRS_ENHANCED
   Intel feature flag also for AutoIBRS.

v1:
 https://lore.kernel.org/kvm/20260224180157.725159-2-kim.phillips@amd.com/

 arch/x86/kernel/cpu/bugs.c   | 12 ++++++++++--
 arch/x86/kernel/cpu/common.c |  6 +-----
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 83f51cab0b1e..957e0df38d90 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2181,7 +2181,14 @@ static void __init spectre_v2_select_mitigation(void)
 			break;
 		fallthrough;
 	case SPECTRE_V2_CMD_FORCE:
-		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
+		/*
+		 * Unless forced, don't use AutoIBRS when SNP is enabled
+		 * because it degrades host userspace indirect branch performance.
+		 */
+		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) &&
+		    (!boot_cpu_has(X86_FEATURE_SEV_SNP) ||
+		     (boot_cpu_has(X86_FEATURE_SEV_SNP) &&
+		      spectre_v2_cmd == SPECTRE_V2_CMD_FORCE))) {
 			spectre_v2_enabled = SPECTRE_V2_EIBRS;
 			break;
 		}
@@ -2261,7 +2268,8 @@ static void __init spectre_v2_apply_mitigation(void)
 
 	case SPECTRE_V2_IBRS:
 		setup_force_cpu_cap(X86_FEATURE_KERNEL_IBRS);
-		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED))
+		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) &&
+		    !boot_cpu_has(X86_FEATURE_SEV_SNP))
 			pr_warn(SPECTRE_V2_IBRS_PERF_MSG);
 		break;
 
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index bb937bc4b00f..5aff1424a27d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1486,13 +1486,9 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	/*
 	 * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel feature
 	 * flag and protect from vendor-specific bugs via the whitelist.
-	 *
-	 * Don't use AutoIBRS when SNP is enabled because it degrades host
-	 * userspace indirect branch performance.
 	 */
 	if ((x86_arch_cap_msr & ARCH_CAP_IBRS_ALL) ||
-	    (cpu_has(c, X86_FEATURE_AUTOIBRS) &&
-	     !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {
+	    cpu_has(c, X86_FEATURE_AUTOIBRS)) {
 		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
 		if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
 		    !(x86_arch_cap_msr & ARCH_CAP_PBRSB_NO))
-- 
2.43.0