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

[Sean Christopherson]

On Mon, Nov 22, 2021, Ben Gardon wrote:
> On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> 
> 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.