Re: [PATCH v4 24/30] KVM: x86/mmu: Zap defunct roots via asynchronous worker

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 03, 2022, Paolo Bonzini wrote:
> Zap defunct roots, a.k.a. roots that have been invalidated after their
> last reference was initially dropped, asynchronously via the system work
> queue instead of forcing the work upon the unfortunate task that happened
> to drop the last reference.
> 
> If a vCPU task drops the last reference, the vCPU is effectively blocked
> by the host for the entire duration of the zap.  If the root being zapped
> happens be fully populated with 4kb leaf SPTEs, e.g. due to dirty logging
> being active, the zap can take several hundred seconds.  Unsurprisingly,
> most guests are unhappy if a vCPU disappears for hundreds of seconds.
> 
> E.g. running a synthetic selftest that triggers a vCPU root zap with
> ~64tb of guest memory and 4kb SPTEs blocks the vCPU for 900+ seconds.
> Offloading the zap to a worker drops the block time to <100ms.
> 
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Ben Gardon <bgardon@google.com>
> Message-Id: <20220226001546.360188-23-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index e24a1bff9218..2456f880508d 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -170,13 +170,24 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  	 */
>  	if (!kvm_tdp_root_mark_invalid(root)) {
>  		refcount_set(&root->tdp_mmu_root_count, 1);
> -		tdp_mmu_zap_root(kvm, root, shared);
>  
>  		/*
> -		 * Give back the reference that was added back above.  We now
> +		 * If the struct kvm is alive, we might as well zap the root
> +		 * in a worker.  The worker takes ownership of the reference we
> +		 * just added to root and is flushed before the struct kvm dies.

Not a fan of the "we might as well zap the root in a worker", IMO we should require
going forward that invalidated, reachable TDP MMU roots are always zapped in a worker

> +		 */
> +		if (likely(refcount_read(&kvm->users_count))) {
> +			tdp_mmu_schedule_zap_root(kvm, root);

Regarding the need for kvm_tdp_mmu_invalidate_all_roots() to guard against
re-queueing a root for zapping, this is the point where it becomes functionally
problematic.  When "fast zap" was the only user of tdp_mmu_schedule_zap_root(),
re-queueing was benign as the work struct was guaranteed to not be currently
queued.  But this code runs outside of slots_lock, and so a root that was "put"
but hasn't finished zapping can be observed and re-queued by the "fast zap.

I think it makes sense to create a rule/invariant that an invalidated TDP MMU root
_must_ be zapped via the work queue.  Then 

I.e. do this as fixup:

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 40bf861b622a..cff4f2102a63 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1019,8 +1019,9 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
  * of invalidated roots; the list is effectively the list of work items in
  * the workqueue.
  *
- * Skip roots that are already queued for zapping, flushing the work queue will
- * ensure invalidated roots are zapped regardless of when they were queued.
+ * Skip roots that are already invalid and thus queued for zapping, flushing
+ * the work queue will ensure invalid roots are zapped regardless of when they
+ * were queued.
  *
  * Because mmu_lock is held for write, it should be impossible to observe a
  * root with zero refcount,* i.e. the list of roots cannot be stale.
@@ -1034,13 +1035,12 @@ void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm)

        lockdep_assert_held_write(&kvm->mmu_lock);
        list_for_each_entry(root, &kvm->arch.tdp_mmu_roots, link) {
-               if (root->tdp_mmu_async_data)
+               if (kvm_tdp_root_mark_invalid(root))
                        continue;

                if (WARN_ON_ONCE(!kvm_tdp_mmu_get_root(root)))
                        continue;

-               root->role.invalid = true;
                tdp_mmu_schedule_zap_root(kvm, root);
        }
 }

> +			return;
> +		}
> +
> +		/*
> +		 * The struct kvm is being destroyed, zap synchronously and give
> +		 * back immediately the reference that was added above.  We now
>  		 * know that the root is invalid, so go ahead and free it if
>  		 * no one has taken a reference in the meanwhile.
>  		 */
> +		tdp_mmu_zap_root(kvm, root, shared);
>  		if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
>  			return;
>  	}
> -- 
> 2.31.1
> 
>