[tip: x86/sev] x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled

[tip-bot2 for Kim Phillips]

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

Commit-ID:     acaa4b5c4c854b5009f4d4a5395b2609ad0f4937
Gitweb:        https://git.kernel.org/tip/acaa4b5c4c854b5009f4d4a5395b2609ad0f4937
Author:        Kim Phillips <kim.phillips@xxxxxxx>
AuthorDate:    Thu, 25 Jan 2024 22:11:02 -06:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Mon, 29 Jan 2024 17:19:01 +01:00

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@xxxxxxx>
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Link: https://lore.kernel.org/r/20240126041126.1927228-3-michael.roth@xxxxxxx
---
 arch/x86/kernel/cpu/common.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0b97bcd..9e35e27 100644
--- 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))