Re: [PATCH v10 05/50] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

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On 10/16/23 15:27, Michael Roth wrote:
From: Kim Phillips <kim.phillips@xxxxxxx>

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. Fall back to retpolines instead.

Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
[mdr: squash in changes from review discussion]
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>

Queued, thanks.

Paolo

---
  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 382d4e6b848d..11fae89b799e 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1357,8 +1357,13 @@ 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))





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