Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Nov 29, 2019 at 04:34:54PM -0500, Peter Xu wrote:
> +static void mark_page_dirty_in_ring(struct kvm *kvm,
> +				    struct kvm_vcpu *vcpu,
> +				    struct kvm_memory_slot *slot,
> +				    gfn_t gfn)
> +{
> +	u32 as_id = 0;

Redundant initialization of as_id.

> +	u64 offset;
> +	int ret;
> +	struct kvm_dirty_ring *ring;
> +	struct kvm_dirty_ring_indexes *indexes;
> +	bool is_vm_ring;
> +
> +	if (!kvm->dirty_ring_size)
> +		return;
> +
> +	offset = gfn - slot->base_gfn;
> +
> +	if (vcpu) {
> +		as_id = kvm_arch_vcpu_memslots_id(vcpu);
> +	} else {
> +		as_id = 0;

The setting of as_id is wrong, both with and without a vCPU.  as_id should
come from slot->as_id.  It may not be actually broken in the current code
base, but at best it's fragile, e.g. Ben's TDP MMU rewrite[*] adds a call
to mark_page_dirty_in_slot() with a potentially non-zero as_id.

[*] https://lkml.kernel.org/r/20190926231824.149014-25-bgardon@google.com

> +		vcpu = kvm_get_running_vcpu();
> +	}
> +
> +	if (vcpu) {
> +		ring = &vcpu->dirty_ring;
> +		indexes = &vcpu->run->vcpu_ring_indexes;
> +		is_vm_ring = false;
> +	} else {
> +		/*
> +		 * Put onto per vm ring because no vcpu context.  Kick
> +		 * vcpu0 if ring is full.
> +		 */
> +		vcpu = kvm->vcpus[0];

Is this a rare event?

> +		ring = &kvm->vm_dirty_ring;
> +		indexes = &kvm->vm_run->vm_ring_indexes;
> +		is_vm_ring = true;
> +	}
> +
> +	ret = kvm_dirty_ring_push(ring, indexes,
> +				  (as_id << 16)|slot->id, offset,
> +				  is_vm_ring);
> +	if (ret < 0) {
> +		if (is_vm_ring)
> +			pr_warn_once("vcpu %d dirty log overflow\n",
> +				     vcpu->vcpu_id);
> +		else
> +			pr_warn_once("per-vm dirty log overflow\n");
> +		return;
> +	}
> +
> +	if (ret)
> +		kvm_make_request(KVM_REQ_DIRTY_RING_FULL, vcpu);
> +}