linux-kernel - [PATCH v1 02/26] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

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Message-ID: <20231230161954.569267-3-michael.roth@amd.com>
Date: Sat, 30 Dec 2023 10:19:30 -0600
From: Michael Roth <michael.roth@....com>
To: <x86@...nel.org>
CC: <kvm@...r.kernel.org>, <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
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	Kim Phillips <kim.phillips@....com>, Dave Hansen <dave.hansen@...el.com>
Subject: [PATCH v1 02/26] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

From: Kim Phillips <kim.phillips@....com>

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@....com>
Acked-by: Dave Hansen <dave.hansen@...el.com>
[mdr: squash in changes from review discussion]
Signed-off-by: Michael Roth <michael.roth@....com>
---
 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 8f367d376520..6b253440ea72 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))
-- 
2.25.1