This is a note to let you know that I've just added the patch titled
x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled
to the 6.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-speculation-do-not-enable-automatic-ibrs-if-sev-snp-is-enabled.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From acaa4b5c4c854b5009f4d4a5395b2609ad0f4937 Mon Sep 17 00:00:00 2001
From: Kim Phillips <kim.phillips@xxxxxxx>
Date: Thu, 25 Jan 2024 22:11:02 -0600
Subject: x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled
From: Kim Phillips <kim.phillips@xxxxxxx>
commit acaa4b5c4c854b5009f4d4a5395b2609ad0f4937 upstream.
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
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/cpu/common.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1385,8 +1385,13 @@ static void __init cpu_set_bug_bits(stru
/*
* 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))
Patches currently in stable-queue which might be from kim.phillips@xxxxxxx are
queue-6.8/x86-speculation-do-not-enable-automatic-ibrs-if-sev-snp-is-enabled.patch
