Re: [PATCH v2 21/21] KVM:x86: Support CET supervisor shadow stack MSR access

[Yang, Weijiang]

On 5/4/2023 12:17 PM, Edgecombe, Rick P wrote:
> On Thu, 2023-05-04 at 09:20 +0800, Yang, Weijiang wrote:
> > On 5/4/2023 1:07 AM, Edgecombe, Rick P wrote:
> > > On Fri, 2023-04-21 at 09:46 -0400, Yang Weijiang wrote:
> > > > +
> > > > +       incpt = !is_cet_state_supported(vcpu,
> > > > XFEATURE_MASK_CET_KERNEL);
> > > > +       incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> > > > +
> > > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> > > > MSR_TYPE_RW, incpt);
> > > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> > > > MSR_TYPE_RW, incpt);
> > > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> > > > MSR_TYPE_RW, incpt);
> > > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> > > > MSR_TYPE_RW, incpt);
> > > >     }
> > > Why is this tied to XFEATURE_MASK_CET_KERNEL? I don't know how the
> > > SVM
> > > side works, but the host kernel doesn't use this xfeature. Just not
> > > clear on what the intention is. Why not use
> > > kvm_cet_kernel_shstk_supported() again?
> > I don't know how SVM supports supervisor SHSTK either, here just
> > follows
> > the spec.
> What aspect of the spec is this?

I assumed the supervisor SHSTK states are backed via XSAVES/SRSTORS with

XFEATURE_MASK_CET_KERNEL set in XSS.  This is arguable since implementation

is not determined, but XSAVES is an efficient way to manage the states 
compared with

manually save/restore the MSRs.

> > to add the dependency check. Maybe you're right, I need to use
> > kvm_cet_kernel_shstk_supported()
> >
> > in my patch set and leave the work to SVM enabling patches. I'll
> > change
> > it, thanks!
> Oh, I see the the SVM patch [0] is adding XFEATURE_MASK_CET_KERNEL to
> kvm_caps.supported_xss as long as kvm_cpu_cap_has(X86_FEATURE_SHSTK).
> And it does not look to be checking XSS host support like how
> kvm_caps.supported_xss is set in your patch. It should depend on host
> support, right?

Yes, it should rely on host to back the states as long as the supervisor

SHSTK MSRs are implemented as XSAVES/XRSTORS managed.

> Is that the intent of kvm_caps.supported_xss?

Yes, it's used to indicate all host XSS supported guest features.

> Separate from all that, the code above is in VMX, so not sure how it
> affects SVM in any case.

I was confused a bit. Yes, the pass-through check is specific to VMX, 
there could

be other implementation in SVM.

> I might be confused here. The code just looked suspicious.
>
> [0]
> https://lore.kernel.org/kvm/20221012203910.204793-8-john.allen@xxxxxxx/

IMO, above patch is not necessary as  kvm_caps.supported_xss is 
initialized in x86 part and

shared by both SVM and VMX.