Re: [PATCH 17/28] KVM: x86/mmu: Terminate yield-friendly walk if invalid root observed

Ben Gardon
2021-11-22T22:25:25Z

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Stop walking TDP MMU roots if the previous root was invalidated while the
> caller yielded (dropped mmu_lock). Any effect that the caller wishes to
> be recognized by a root must be made visible before walking the list of
> roots. Because all roots are invalided when any root is invalidated, if
> the previous root was invalidated, then all roots on the list when the
> caller first started the walk also were invalidated, and any valid roots
> on the list must have been added after the invalidation event.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 19 ++++++++++++-------
> 1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index d9524b387221..cc8d021a1ba5 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -140,16 +140,21 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> lockdep_assert_held(&kvm->mmu_lock);
>
> /*
> - * Restart the walk if the previous root was invalidated, which can
> - * happen if the caller drops mmu_lock when yielding. Restarting the
> - * walke is necessary because invalidating a root also removes it from
> - * tdp_mmu_roots. Restarting is safe and correct because invalidating
> - * a root is done if and only if _all_ roots are invalidated, i.e. any
> - * root on tdp_mmu_roots was added _after_ the invalidation event.
> + * Terminate the walk if the previous root was invalidated, which can
> + * happen if the caller yielded and dropped mmu_lock. Because invalid
> + * roots are removed from tdp_mmu_roots with mmu_lock held for write,
> + * if the previous root was invalidated, then the invalidation occurred
> + * after this walk started. And because _all_ roots are invalidated
> + * during an invalidation event, any root on tdp_mmu_roots was created
> + * after the invalidation. Lastly, any state change being made by the
> + * caller must be effected before updating SPTEs, otherwise vCPUs could
> + * simply create new SPTEs with the old state. Thus, if the previous
> + * root was invalidated, all valid roots are guaranteed to have been
> + * created after the desired state change and don't need to be updated.
> */
> if (prev_root && prev_root->role.invalid) {
> kvm_tdp_mmu_put_root(kvm, prev_root, shared);
> - prev_root = NULL;
> + return NULL;
> }

Another option might be to walk the list FIFO so that the newly
created roots are encountered, but since they start empty, there might
not be any work to do.

>
> /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>