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

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Date:   Wed, 4 Dec 2019 18:38:35 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking


On 2019/11/30 上午5:34, Peter Xu wrote:
> +int kvm_dirty_ring_push(struct kvm_dirty_ring *ring,
> +			struct kvm_dirty_ring_indexes *indexes,
> +			u32 slot, u64 offset, bool lock)
> +{
> +	int ret;
> +	struct kvm_dirty_gfn *entry;
> +
> +	if (lock)
> +		spin_lock(&ring->lock);
> +
> +	if (kvm_dirty_ring_full(ring)) {
> +		ret = -EBUSY;
> +		goto out;
> +	}
> +
> +	entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> +	entry->slot = slot;
> +	entry->offset = offset;


Haven't gone through the whole series, sorry if it was a silly question 
but I wonder things like this will suffer from similar issue on 
virtually tagged archs as mentioned in [1].

Is this better to allocate the ring from userspace and set to KVM 
instead? Then we can use copy_to/from_user() friends (a little bit slow 
on recent CPUs).

[1] https://lkml.org/lkml/2019/4/9/5

Thanks


> +	smp_wmb();
> +	ring->dirty_index++;
> +	WRITE_ONCE(indexes->avail_index, ring->dirty_index);
> +	ret = kvm_dirty_ring_used(ring) >= ring->soft_limit;
> +	pr_info("%s: slot %u offset %llu used %u\n",
> +		__func__, slot, offset, kvm_dirty_ring_used(ring));
> +
> +out: