On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > +static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,
> > + unsigned int x86_feature)
> > {
> > const struct cpuid_reg cpuid = x86_feature_cpuid(x86_feature);
> > struct kvm_cpuid_entry2 *entry;
> > + u32 *reg;
> > +
> > + /*
> > + * XSAVES is a special snowflake. Due to lack of a dedicated intercept
> > + * on SVM, KVM must assume that XSAVES (and thus XRSTORS) is usable by
> > + * the guest if the host supports XSAVES and *XSAVE* is exposed to the
> > + * guest. Although the guest can read/write XSS via XSAVES/XRSTORS, to
> > + * minimize the virtualization hole, KVM rejects attempts to read/write
> > + * XSS via RDMSR/WRMSR. To make that work, KVM needs to check the raw
> > + * guest CPUID, not KVM's view of guest capabilities.
>
> Hi,
>
> I think that this comment is wrong:
>
> The guest can't read/write XSS via XSAVES/XRSTORS. It can only use XSAVES/XRSTORS
> to save/restore features that are enabled in XSS, and thus if there are none enabled,
> the XSAVES/XRSTORS acts as more or less XSAVEOPTC/XRSTOR except working only when CPL=0)
Doh, right you are.
> So I don't think that there is a virtualization hole except the fact that VMM can't
> really disable XSAVES if it chooses to.
There is still a hole. If XSAVES is not supported, KVM runs the guest with the
host XSS. See the conditional switching in kvm_load_{guest,host}_xsave_state().
Not treating XSAVES as being available to the guest would allow the guest to read
and write host supervisor state.
I'll rewrite the comment to call that.
> Another "half virtualization hole" is that since we have chosen to not
> intercept XSAVES at all, (AMD can't do this at all, and it's slow anyway) we
> instead opted to never support some XSS bits (so far all of them, only
> upcoming CET will add a few supported bits).
>
> This creates an unexpected situation for the guest - enabled feature (e.g PT)
> but no XSS bit supported to context switch it. x86 arch does allow this
> though.
