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

[Yang, Weijiang]

On 8/4/2023 1:51 PM, 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?
Hold on until host_initiated access is finalized.
> > > > +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().
OK, maybe the helper is not necessary anymore, I will remove it, thank you!
> > kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)
> >
> > only tells at least one of the CET features is supported by KVM.
> >
> > > then patch 16 has no need to do
> > >
> > > +	/*
> > > +	 * If SHSTK and IBT are not available in KVM, clear CET user bit in
> > > +	 * kvm_caps.supported_xss so that kvm_is_cet__supported() returns
> > > +	 * false when called.
> > > +	 */
> > > +	if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
> > > +	    !kvm_cpu_cap_has(X86_FEATURE_IBT))
> > > +		kvm_caps.supported_xss &= ~CET_XSTATE_MASK;