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.
