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Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking
To:     Peter Xu <peterx@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
References: <20191129213505.18472-1-peterx@redhat.com>
 <20191129213505.18472-5-peterx@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <1355422f-ab62-9dc3-2b48-71a6e221786b@redhat.com>
Date:   Wed, 4 Dec 2019 18:38:35 +0800
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On 2019/11/30 =E4=B8=8A=E5=8D=885:34, Peter Xu wrote:
> +int kvm_dirty_ring_push(struct kvm_dirty_ring *ring,
> +=09=09=09struct kvm_dirty_ring_indexes *indexes,
> +=09=09=09u32 slot, u64 offset, bool lock)
> +{
> +=09int ret;
> +=09struct kvm_dirty_gfn *entry;
> +
> +=09if (lock)
> +=09=09spin_lock(&ring->lock);
> +
> +=09if (kvm_dirty_ring_full(ring)) {
> +=09=09ret =3D -EBUSY;
> +=09=09goto out;
> +=09}
> +
> +=09entry =3D &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> +=09entry->slot =3D slot;
> +=09entry->offset =3D offset;


Haven't gone through the whole series, sorry if it was a silly question=20
but I wonder things like this will suffer from similar issue on=20
virtually tagged archs as mentioned in [1].

Is this better to allocate the ring from userspace and set to KVM=20
instead? Then we can use copy_to/from_user() friends (a little bit slow=20
on recent CPUs).

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

Thanks


> +=09smp_wmb();
> +=09ring->dirty_index++;
> +=09WRITE_ONCE(indexes->avail_index, ring->dirty_index);
> +=09ret =3D kvm_dirty_ring_used(ring) >=3D ring->soft_limit;
> +=09pr_info("%s: slot %u offset %llu used %u\n",
> +=09=09__func__, slot, offset, kvm_dirty_ring_used(ring));
> +
> +out: