x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled

Without SEV-SNP, Automatic IBRS protects only the kernel. But when
SEV-SNP is enabled, the Automatic IBRS protection umbrella widens to all
host-side code, including userspace. This protection comes at a cost:
reduced userspace indirect branch performance.

To avoid this performance loss, don't use Automatic IBRS on SEV-SNP
hosts and all back to retpolines instead.

  [ mdr: squash in changes from review discussion. ]

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/r/20240126041126.1927228-3-michael.roth@amd.com

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0b97bcde70c6102a4b82b561c3256ec53b614770..9e35e276c55aa8048a313db277bbee5496b2223c 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1355,8 +1355,13 @@ 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 ((ia32_cap & ARCH_CAP_IBRS_ALL) || cpu_has(c, X86_FEATURE_AUTOIBRS)) {
+       if ((ia32_cap & ARCH_CAP_IBRS_ALL) ||
+           (cpu_has(c, X86_FEATURE_AUTOIBRS) &&
+            !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {
                setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
                if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
                    !(ia32_cap & ARCH_CAP_PBRSB_NO))