On Fri, Aug 23, 2024 at 3:12 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > > On 8/23/24 15:51, Sean Christopherson wrote: > > On Fri, Aug 23, 2024, Tom Lendacky wrote: > >> On 8/23/24 13:53, Jim Mattson wrote: > >>> From Intel's documention [1], "CPUID.(EAX=07H,ECX=0):EDX[26] > >>> enumerates support for indirect branch restricted speculation (IBRS) > >>> and the indirect branch predictor barrier (IBPB)." Further, from [2], > >>> "Software that executed before the IBPB command cannot control the > >>> predicted targets of indirect branches (4) executed after the command > >>> on the same logical processor," where footnote 4 reads, "Note that > >>> indirect branches include near call indirect, near jump indirect and > >>> near return instructions. Because it includes near returns, it follows > >>> that **RSB entries created before an IBPB command cannot control the > >>> predicted targets of returns executed after the command on the same > >>> logical processor.**" [emphasis mine] > >>> > >>> On the other hand, AMD's IBPB "may not prevent return branch > >>> predictions from being specified by pre-IBPB branch targets" [3]. > >>> > >>> However, some AMD processors have an "enhanced IBPB" [terminology > >>> mine] which does clear the return address predictor. This feature is > >>> enumerated by CPUID.80000008:EDX.IBPB_RET[bit 30] [4]. > >>> > >>> Adjust the cross-vendor features enumerated by KVM_GET_SUPPORTED_CPUID > >>> accordingly. > >>> > >>> [1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/cpuid-enumeration-and-architectural-msrs.html > >>> [2] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/speculative-execution-side-channel-mitigations.html#Footnotes > >>> [3] https://www.amd.com/en/resources/product-security/bulletin/amd-sb-1040.html > >>> [4] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24594.pdf > >>> > >>> Fixes: 0c54914d0c52 ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code") > >>> Suggested-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxxxx> > >>> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx> > >>> --- > >>> arch/x86/kvm/cpuid.c | 6 +++++- > >>> 1 file changed, 5 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > >>> index ec7b2ca3b4d3..c8d7d928ffc7 100644 > >>> --- a/arch/x86/kvm/cpuid.c > >>> +++ b/arch/x86/kvm/cpuid.c > >>> @@ -690,7 +690,9 @@ void kvm_set_cpu_caps(void) > >>> kvm_cpu_cap_set(X86_FEATURE_TSC_ADJUST); > >>> kvm_cpu_cap_set(X86_FEATURE_ARCH_CAPABILITIES); > >>> > >>> - if (boot_cpu_has(X86_FEATURE_IBPB) && boot_cpu_has(X86_FEATURE_IBRS)) > >>> + if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) && > >>> + boot_cpu_has(X86_FEATURE_AMD_IBPB) && > >>> + boot_cpu_has(X86_FEATURE_AMD_IBRS)) > >>> kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL); > >>> if (boot_cpu_has(X86_FEATURE_STIBP)) > >>> kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP); > >>> @@ -759,6 +761,8 @@ void kvm_set_cpu_caps(void) > >>> * arch/x86/kernel/cpu/bugs.c is kind enough to > >>> * record that in cpufeatures so use them. > >>> */ > >>> + if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) > >>> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > >> > >> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > >> without AMD_IBPB, but it just looks odd seeing them set with separate > >> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > >> That's just me, though, not worth a v4 unless others feel the same. > > > > You thinking something like this (at the end, after the dust settles)? > > > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > I was just thinking more along the lines of: > > if (boot_cpu_has(X86_FEATURE_IBPB)) { > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > } AFAICT, there are just two reasons that X86_FEATURE_IBPB gets set: 1. The CPU reports CPUID.(EAX=7,ECX=0):EDX[bit 26] (aka X86_FEATURE_SPEC_CTRL) 2. The CPU reports CPUID Fn8000_0008_EBX[IBPB] (aka X86_FEATURE_AMD_IBPB) Clearly, in the second case, the KVM cpu capability for AMD_IBPB will already be set, since it's specified in the mask for CPUID_8000_0008_EBX. If this block of code is just trying to populate CPUID Fn8000_0008_EBX on Intel processors, I'd rather change all of the predicates to test for Intel features, rather than vendor-neutral features, so that the derivation is clear. But maybe this block of code is also trying to populate CPUID Fn8000_0008_EBX on AMD processors that may have some of these features, but don't enumerate them via CPUID? > Thanks, > Tom > > >> > > > >> Thanks, > >> Tom > >> > >>> if (boot_cpu_has(X86_FEATURE_IBPB)) > >>> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > >>> if (boot_cpu_has(X86_FEATURE_IBRS))
