[PATCH v3 2/6] cpu/bugs: Allow spectre_v2=ibrs on x86 vendors other than Intel

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

This is to prepare to allow legacy IBRS toggling on AMD systems,
where the BTB Isolation SEV-SNP feature can use it to optimize the
quick VM exit to re-entry path.

There is no reason this wasn't allowed in the first place, therefore
adding the cc: stable and Fixes: tags.

Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@kernel.org
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
v3: No changes
v2: No changes
    https://lore.kernel.org/kvm/20260311130611.2201214-3-kim.phillips@amd.com/
v1: https://lore.kernel.org/kvm/20260224180157.725159-3-kim.phillips@amd.com/

 arch/x86/kernel/cpu/bugs.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dfefbde10646..eed5a72a870c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2153,11 +2153,6 @@ static void __init spectre_v2_select_mitigation(void)
 		spectre_v2_cmd = SPECTRE_V2_CMD_AUTO;
 	}
 
-	if (spectre_v2_cmd == SPECTRE_V2_CMD_IBRS && boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
-		pr_err("IBRS selected but not Intel CPU. Switching to AUTO select\n");
-		spectre_v2_cmd = SPECTRE_V2_CMD_AUTO;
-	}
-
 	if (spectre_v2_cmd == SPECTRE_V2_CMD_IBRS && !boot_cpu_has(X86_FEATURE_IBRS)) {
 		pr_err("IBRS selected but CPU doesn't have IBRS. Switching to AUTO select\n");
 		spectre_v2_cmd = SPECTRE_V2_CMD_AUTO;
@@ -2250,7 +2245,8 @@ static void __init spectre_v2_apply_mitigation(void)
 		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
 
 	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled)) {
-		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS) &&
+		    spectre_v2_enabled != SPECTRE_V2_IBRS) {
 			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
 		} else {
 			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-- 
2.43.0