[PATCH v3 2/2] x86: KVM: Advertise AMD's speculation control features

Maksim Davydov
2024-12-04T13:45:51Z

It seems helpful to expose to userspace some speculation control features
from 0x80000008_EBX function:
* 16 bit. IBRS always on. Indicates whether processor prefers that
IBRS is always on. It simplifies speculation managing.
* 18 bit. IBRS is preferred over software solution. Indicates that
software mitigations can be replaced with more performant IBRS.
* 19 bit. IBRS provides Same Mode Protection. Indicates that when IBRS
is set indirect branch predictions are not influenced by any prior
indirect branches.
* 29 bit. BTC_NO. Indicates that processor isn't affected by branch type
confusion. It's used during mitigations setting up.

Signed-off-by: Maksim Davydov <davydov-max@yandex-team.ru>
Reviewed-by: Jim Mattson <jmattson@google.com>
---
arch/x86/include/asm/cpufeatures.h | 3 +++
arch/x86/kvm/cpuid.c | 5 +++--
2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 45f87a026bba..0a709d03ee5c 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -343,7 +343,10 @@
#define X86_FEATURE_AMD_IBPB (13*32+12) /* Indirect Branch Prediction Barrier */
#define X86_FEATURE_AMD_IBRS (13*32+14) /* Indirect Branch Restricted Speculation */
#define X86_FEATURE_AMD_STIBP (13*32+15) /* Single Thread Indirect Branch Predictors */
+#define X86_FEATURE_AMD_IBRS_ALWAYS_ON (13*32+16) /* Indirect Branch Restricted Speculation always-on preferred */
#define X86_FEATURE_AMD_STIBP_ALWAYS_ON (13*32+17) /* Single Thread Indirect Branch Predictors always-on preferred */
+#define X86_FEATURE_AMD_IBRS_PREFERRED (13*32+18) /* Indirect Branch Restricted Speculation is preferred over SW solution */
+#define X86_FEATURE_AMD_IBRS_SAME_MODE (13*32+19) /* Indirect Branch Restricted Speculation provides Same Mode protection */
#define X86_FEATURE_AMD_PPIN (13*32+23) /* "amd_ppin" Protected Processor Inventory Number */
#define X86_FEATURE_AMD_SSBD (13*32+24) /* Speculative Store Bypass Disable */
#define X86_FEATURE_VIRT_SSBD (13*32+25) /* "virt_ssbd" Virtualized Speculative Store Bypass Disable */
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 7bc095add8ee..204192425e2c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -756,8 +756,9 @@ void kvm_set_cpu_caps(void)
kvm_cpu_cap_mask(CPUID_8000_0008_EBX,
F(CLZERO) | F(XSAVEERPTR) |
F(WBNOINVD) | F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
- F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON) |
- F(AMD_PSFD) | F(AMD_IBPB_RET)
+ F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_IBRS_ALWAYS_ON) |
+ F(AMD_STIBP_ALWAYS_ON) | F(AMD_IBRS_PREFERRED) |
+ F(AMD_IBRS_SAME_MODE) | F(AMD_PSFD) | F(BTC_NO) | F(AMD_IBPB_RET)
);

/*
--
2.34.1