Re: [RFC PATCH v5 089/104] KVM: TDX: Add a placeholder for handler of TDX hypercalls (TDG.VP.VMCALL)

[Paolo Bonzini]

On 4/7/22 16:39, Sean Christopherson wrote:
> On Thu, Apr 07, 2022, Paolo Bonzini wrote:
> > On 4/7/22 06:15, Kai Huang wrote:
> > > > +static int handle_tdvmcall(struct kvm_vcpu *vcpu)
> > > > +{
> > > > +	struct vcpu_tdx *tdx = to_tdx(vcpu);
> > > > +
> > > > +	if (unlikely(tdx->tdvmcall.xmm_mask))
> > > > +		goto unsupported;
> > > Put a comment explaining this logic?
> >
> > This only seems to be necessary for Hyper-V hypercalls, which however are
> > not supported by this series in TDX guests (because the kvm_hv_hypercall
> > still calls kvm_*_read, likewise for Xen).
> >
> > So for now this conditional can be dropped.
>
> I'd prefer to keep the sanity check, it's a cheap and easy way to detect a clear
> cut guest bug.

I don't think it's necessarily a guest bug, just silly but valid behavior.

Paolo

  E.g. KVM would be within its rights to write garbage the XMM
> registers in this case.  Even though KVM isn't to be trusted, KVM can still be
> nice to the guest.