Re: [PATCH v10 035/108] KVM: x86/mmu: Track shadow MMIO value on a per-VM basis

Yan Zhao
2022-11-25T01:00:28Z

On Fri, Nov 25, 2022 at 08:45:01AM +0800, Huang, Kai wrote:
> On Fri, 2022-11-25 at 08:12 +0800, Yan Zhao wrote:
> > On Fri, Nov 25, 2022 at 08:13:48AM +0800, Huang, Kai wrote:
> > > On Tue, 2022-11-22 at 10:10 +0800, Yan Zhao wrote:
> > > > Also make enable_mmio_caching to be a per-VM value?
> > > > As if the shadow_mmio_value is 0, mmio_caching needs to be disabled.
> > >
> > > If I recall correctly, Sean said we can disable TDX guests if mmio_caching is
> > > disabled (we also will need to change to allow enable_mmio_caching to still be
> > > true when mmio_value is 0).
> > >
> > > SEV_ES has similar logic:
> > >
> > > void __init sev_hardware_setup(void)
> > > {
> > >
> > > ...
> > >
> > > /*
> > > * SEV-ES requires MMIO caching as KVM doesn't have access to the guest
> > > * instruction stream, i.e. can't emulate in response to a #NPF and
> > > * instead relies on #NPF(RSVD) being reflected into the guest as #VC
> > > * (the guest can then do a #VMGEXIT to request MMIO emulation).
> > > */
> > > if (!enable_mmio_caching)
> > > goto out;
> > >
> >
> > Would enabling mmio caching in per-VM basis be better?
> >
>
> We need Paolo/Sean to decide.
>
> The thing is TDX guests always require mmio_caching being enabled. For VMX
> guests, normally we will always enable mmio_caching too. So I think per-VM
> basis mmio_caching is not that useful.
With per-VM basis enabling, I think we can get rid of the kvm_gfn_shared_mask(kvm)
in below code and also in handle_abnormal_pfn()

static inline bool is_mmio_spte(struct kvm *kvm, u64 spte)
{
return (spte & shadow_mmio_mask) == kvm->arch.shadow_mmio_value &&
likely(enable_mmio_caching || kvm_gfn_shared_mask(kvm));
}

Thanks
Yan