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. Thanks, Tom > if (boot_cpu_has(X86_FEATURE_IBPB)) > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > if (boot_cpu_has(X86_FEATURE_IBRS))
