On Fri, 2022-11-25 at 08:37 +0800, Yan Zhao wrote:
> 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));
> }
>
It needs to go anyway regardless per-VM mmio_caching or not, as explained we
need to change to allow enable_mmio_caching to be true even mmio_value is 0.
