On Wed, Sep 01, 2021, Sean Christopherson wrote:
> > -static u64 get_ghcb_msr_bits(struct vcpu_svm *svm, u64 mask, unsigned int pos)
> > -{
> > - return (svm->vmcb->control.ghcb_gpa >> pos) & mask;
> > + msr = GHCB_MSR_CPUID_RESP;
> > + msr |= (reg & GHCB_MSR_CPUID_REG_MASK) << GHCB_MSR_CPUID_REG_POS;
> > + msr |= (value & GHCB_MSR_CPUID_VALUE_MASK) << GHCB_MSR_CPUID_VALUE_POS;
> > +
> > + svm->vmcb->control.ghcb_gpa = msr;
>
> I would rather have the get/set pairs be roughly symmetric, i.e. both functions
> or both macros, and both work on svm->vmcb->control.ghcb_gpa or both be purely
> functional (that may not be the correct word).
>
> I don't have a strong preference on function vs. macro. But for the second one,
> my preference would be to have the helper generate the value as opposed to taken
> and filling a pointer, e.g. to yield something like:
>
> cpuid_reg = GHCB_MSR_CPUID_REG(control->ghcb_gpa);
>
> if (cpuid_reg == 0)
> cpuid_value = vcpu->arch.regs[VCPU_REGS_RAX];
> else if (cpuid_reg == 1)
> cpuid_value = vcpu->arch.regs[VCPU_REGS_RBX];
> else if (cpuid_reg == 2)
> cpuid_value = vcpu->arch.regs[VCPU_REGS_RCX];
> else
> cpuid_value = vcpu->arch.regs[VCPU_REGS_RDX];
>
> control->ghcb_gpa = MAKE_GHCB_MSR_RESP(cpuid_reg, cpuid_value);
>
>
> The advantage is that it's obvious from the code that control->ghcb_gpa is being
> read _and_ written.
Ah, but in the next path I see there's the existing ghcb_set_sw_exit_info_2().
Hrm. I think I still prefer open coding "control->ghcb_gpa = ..." with the right
hand side being a macro. That would gel with the INFO_REQ, e.g.
case GHCB_MSR_SEV_INFO_REQ:
control->ghcb_gpa = GHCB_MSR_SEV_INFO(GHCB_VERSION_MAX,
GHCB_VERSION_MIN,
sev_enc_bit));
break;
and drop set_ghcb_msr() altogether.
Side topic, what about renaming control->ghcb_gpa => control->ghcb_msr so that
the code for the MSR protocol is a bit more self-documenting? The APM defines
the field as "Guest physical address of GHCB", so it's not exactly prescribing a
specific name.
