Re: [PATCH 1/2] KVM: Stop KVM_PRE_FAULT_MEMORY when the vCPU's dirty ring is full

[Sean Christopherson]

On Tue, Nov 10, 2026, Fuad Tabba wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cf4cb586f1407..da1b76ceb65e3 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4461,6 +4461,12 @@ static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>  			break;
>  		}
>  
> +		/* Like KVM_RUN: only the iteration crossing the soft limit uses the reserve. */

Please don't include AI comments verbatim.

> +		if (vcpu->kvm->dirty_ring_size && kvm_dirty_ring_soft_full(&vcpu->dirty_ring)) {

Eh, I vote to just use kvm_dirty_ring_check_request(), I don't see any harm in
clobbering vcpu->run->exit_reason.

Side topic, kvm_dirty_ring_check_request() is rather confusing (clears and then
re-sets the requests).  AFAICT, it can be this instead:

	lockdep_assert_held(vcpu->mutex);

	/*
	 * The vCPU isn't runnable when the dirty ring becomes soft full.  Keep
	 * the requests set until the dirty pages are harvested and the dirty
	 * ring is reset by userspace.  Note, checking if the ring is soft-full
	 * can get a false positive, but not a false negative, because only the
	 * vCPU can add entries to the ring, and its mutex is held.
	 */
	if (kvm_test_request(KVM_REQ_DIRTY_RING_SOFT_FULL, vcpu) &&
	    kvm_dirty_ring_soft_full(&vcpu->dirty_ring)) {
		vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
		trace_kvm_dirty_ring_exit(vcpu);
		return true;
	}

	kvm_clear_request(KVM_REQ_DIRTY_RING_SOFT_FULL, vcpu);
	return false;

> +			r = -ENOSPC;
> +			break;
> +		}
> +
>  		r = kvm_arch_vcpu_pre_fault_memory(vcpu, range);
>  		if (WARN_ON_ONCE(r == 0 || r == -EIO))
>  			break;
> -- 
> 2.47.3
>