On Thu, Apr 11, 2024 at 6:32 PM Venkatesh Srinivas
<venkateshs@xxxxxxxxxxxx> wrote:
>
> On Thu, Apr 11, 2024 at 1:59 PM Jim Mattson <jmattson@xxxxxxxxxx> 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].
> >
> > Since Linux sets the synthetic feature bit, X86_FEATURE_IBPB, on AMD
> > CPUs that implement the weaker version of IBPB, it is incorrect to
> > infer from this and X86_FEATURE_IBRS that the CPU supports the
> > stronger version of IBPB indicated by CPUID.(EAX=07H,ECX=0):EDX[26].
>
> AMD's IBPB does apply to RET predictions if Fn8000_0008_EBX[IBPB_RET] = 1.
> Spot checking, Zen4 sets that bit; and the bulletin doesn't apply there.
So, with a definition of X86_FEATURE_AMD_IBPB_RET, this could be:
if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) &&
boot_cpu_has(X86_FEATURE_IBRS))
kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL);
And, in the other direction,
if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);
But, perhaps all of this cross-vendor equivalence logic belongs in user space.
> (Also checking - IA32_SPEC_CTRL and IA32_PRED_CMD are both still
> available; is there anything in KVM that keys off just X86_FEATURE_SPEC_CTRL?
> I'm not seeing it...)
I hope not. It looks like all of the guest_cpuid checks for SPEC_CTRL
also check for the AMD bits (e.g. guest_has_spec_ctrl_msr()).
