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

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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
This commit is contained in:
Kim Phillips
2024-01-25 22:11:02 -06:00
committed by Borislav Petkov (AMD)
parent b6e0f6666f
commit acaa4b5c4c
1 changed files with 6 additions and 1 deletions
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7
arch/x86/kernel/cpu/common.c
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@@ -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))