Re: [PATCH v5 15/19] KVM:x86: Optimize CET supervisor SSP save/reload

[Yang, Weijiang]

On 8/4/2023 4:43 PM, Chao Gao wrote:
> On Thu, Aug 03, 2023 at 12:27:28AM -0400, Yang Weijiang wrote:
> > Make PL{0,1,2}_SSP as write-intercepted to detect whether
> > guest is using these MSRs. Disable intercept to the MSRs
> > if they're written with non-zero values. KVM does save/
> > reload for the MSRs only if they're used by guest.
> What would happen if guest tries to use XRSTORS to load S_CET state from a
> xsave area without any writes to the PL0-2_SSP (i.e., at that point, writes to
> the MSRs are still intercepted)?
I need to do some experiments to get the details, but now expect some kind
of error in guest is seen.
> > @@ -2420,6 +2432,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > 		else
> > 			vmx->pt_desc.guest.addr_a[index / 2] = data;
> > 		break;
> > +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL2_SSP:
> > +		if (kvm_set_msr_common(vcpu, msr_info))
> > +			return 1;
> > +		if (data) {
> > +			vmx_disable_write_intercept_sss_msr(vcpu);
> > +			wrmsrl(msr_index, data);
> Is it necessary to do the wrmsl()?
> looks the next kvm_x86_prepare_switch_to_guest() will load PL0-2_SSP from the
> caching values.
Oh, yes, it's not necessary after moving the reload logic to kvm_x86_prepare_switch_to_guest().
Thanks!