Re: [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root

Ben Gardon
2021-11-22T23:04:04Z

On Mon, Nov 22, 2021 at 2:40 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Nov 22, 2021, Ben Gardon wrote:
> > On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > Allow yielding when zapping SPTEs for a defunct TDP MMU root. Yielding
> > > is safe from a TDP perspective, as the root is unreachable. The only
> > > potential danger is putting a root from a non-preemptible context, and
> > > KVM currently does not do so.
> > >
> > > Yield-unfriendly iteration uses for_each_tdp_mmu_root(), which doesn't
> > > take a reference to each root (it requires mmu_lock be held for the
> > > entire duration of the walk).
> > >
> > > tdp_mmu_next_root() is used only by the yield-friendly iterator.
> > >
> > > kvm_tdp_mmu_zap_invalidated_roots() is explicitly yield friendly.
> > >
> > > kvm_mmu_free_roots() => mmu_free_root_page() is a much bigger fan-out,
> > > but is still yield-friendly in all call sites, as all callers can be
> > > traced back to some combination of vcpu_run(), kvm_destroy_vm(), and/or
> > > kvm_create_vm().
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> >
> > Reviewed-by: Ben Gardon <bgardon@google.com>
> >
> > I'm glad to see this fixed. I assume we don't usually hit this in
> > testing because most of the teardown happens in the zap-all path when
> > we unregister for MMU notifiers
>
> Or more likely, when the userspace process exits and kvm_mmu_notifier_ops.release
> is invoked. But yeah, same difference, VM teardown is unlikely to trigger zapping
> by putting the last TDP MMU reference.
>
> > and actually deleting a fully populated root while the VM is running is pretty
> > rare.
>
> Hmm, probably not that rare, e.g. guest reboot (emulated RESET) is all but
> guaranteed to trigger kvm_mmu_reset_context() on all vCPUs and thus drop all roots,
> and QEMU at least doesn't (always) do memslot updates as part of reboot.

I don't think we have a selftest or kvm-unit-test that builds a large
EPT structure and then reboots the guest though. That'd be a cool test
to have.