Re: [PATCH v5 08/19] KVM:x86: Report KVM supported CET MSRs as to-be-saved

[Yang, Weijiang]

On 8/5/2023 2:51 AM, Sean Christopherson wrote:
> On Fri, Aug 04, 2023, Chao Gao wrote:
> > On Fri, Aug 04, 2023 at 11:13:36AM +0800, Yang, Weijiang wrote:
> > > > > @@ -7214,6 +7217,13 @@ static void kvm_probe_msr_to_save(u32 msr_index)
> > > > > 		if (!kvm_caps.supported_xss)
> > > > > 			return;
> > > > > 		break;
> > > > > +	case MSR_IA32_U_CET:
> > > > > +	case MSR_IA32_S_CET:
> > > > > +	case MSR_KVM_GUEST_SSP:
> > > > > +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> > > > > +		if (!kvm_is_cet_supported())
> > > > shall we consider the case where IBT is supported while SS isn't
> > > > (e.g., in L1 guest)?
> > > Yes, but userspace should be able to access SHSTK MSRs even only IBT is exposed to guest so
> > > far as KVM can support SHSTK MSRs.
> > Why should userspace be allowed to access SHSTK MSRs in this case? L1 may not
> > even enumerate SHSTK (qemu removes -shstk explicitly but keeps IBT), how KVM in
> > L1 can allow its userspace to do that?
> +1.  And specifically, this isn't about SHSTK being exposed to the guest, it's about
> SHSTK being _supported by KVM_.  This is all about KVM telling userspace what MSRs
> are valid and/or need to be saved+restored.  If KVM doesn't support a feature,
> then the MSRs are invalid and there is no reason for userspace to save+restore
> the MSRs on live migration.
OK, will use kvm_cpu_cap_has() to check KVM support before add CET MSRs to the lists.
> > > > > +static inline bool kvm_is_cet_supported(void)
> > > > > +{
> > > > > +	return (kvm_caps.supported_xss & CET_XSTATE_MASK) == CET_XSTATE_MASK;
> > > > why not just check if SHSTK or IBT is supported explicitly, i.e.,
> > > >
> > > > 	return kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
> > > > 	       kvm_cpu_cap_has(X86_FEATURE_IBT);
> > > >
> > > > this is straightforward. And strictly speaking, the support of a feature and
> > > > the support of managing a feature's state via XSAVE(S) are two different things.x
> > > I think using exiting check implies two things:
> > > 1. Platform/KVM can support CET features.
> > > 2. CET user mode MSRs are backed by host thus are guaranteed to be valid.
> > > i.e., the purpose is to check guest CET dependencies instead of features' availability.
> > When KVM claims a feature is supported, it should ensure all its dependencies are
> > met. that's, KVM's support of a feature also imples all dependencies are met.
> > Function-wise, the two approaches have no difference. I just think checking
> > KVM's support of SHSTK/IBT is more clear because the function name is
> > kvm_is_cet_supported() rather than e.g., kvm_is_cet_state_managed_by_xsave().
> +1, one of the big reasons kvm_cpu_cap_has() came about was being KVM had a giant
> mess of one-off helpers.
I see, thanks!