Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM on Hyper-V

[Paolo Bonzini]

On 2/24/23 17:17, Jeremi Piotrowski wrote:
> > > > > > > Depending on the performance results of adding the hypercall to
> > > > > > > svm_flush_tlb_current, the fix could indeed be to just disable usage of
> > > > > > > HV_X64_NESTED_ENLIGHTENED_TLB.
> > > > > > Minus making nested SVM (L3) mutually exclusive, I believe this will do the trick:
> > > > > >
> > > > > > +       /* blah blah blah */
> > > > > > +       hv_flush_tlb_current(vcpu);
> > > > > > +
> > > > > Yes, it's either this or disabling the feature.
> > > > >
> > > > > Paolo
> > > > Combining the two sub-threads: both of the suggestions:
> > > >
> > > > a) adding a hyperv_flush_guest_mapping(__pa(root->spt) after kvm_tdp_mmu_get_vcpu_root_hpa's call to tdp_mmu_alloc_sp()
> > > > b) adding a hyperv_flush_guest_mapping(vcpu->arch.mmu->root.hpa) to svm_flush_tlb_current()
> > > >
> > > > appear to work in my test case (L2 vm startup until panic due to missing rootfs).
> > > >
> > > > But in both these cases (and also when I completely disable HV_X64_NESTED_ENLIGHTENED_TLB)
> > > > the runtime of an iteration of the test is noticeably longer compared to tdp_mmu=0.
> > > Hmm, what is test doing?
> > Booting through OVMF and kernel with no rootfs provided, and panic=-1 specified on the
> > kernel command line. It's a pure startup time test.
> Hi Sean,
>
> Have you been able to reproduce this by any chance?
>
> I would be glad to see either of the two fixes getting merged (b) or a) if it doesn't require
> special L3 nested handling) in order to get this regression resolved.

For now the easiest course of action is to just disable the TDP MMU if 
HV_X64_NESTED_ENLIGHTENED_TLB is available.

Paolo