2024-01-26 04:51:19

by Michael Roth

[permalink] [raw]
Subject: [PATCH v2 02/25] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

From: Kim Phillips <[email protected]>

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 <[email protected]>
Acked-by: Dave Hansen <[email protected]>
[mdr: squash in changes from review discussion]
Signed-off-by: Michael Roth <[email protected]>
---
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



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

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 <[email protected]>
AuthorDate: Thu, 25 Jan 2024 22:11:02 -06:00
Committer: Borislav Petkov (AMD) <[email protected]>
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 <[email protected]>
Signed-off-by: Michael Roth <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
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))