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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Sat, 21 May 2016 17:37:27 +0200
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Subject: Re: [Qemu-devel] [PATCH 7/9] virtio-blk: live migrate s->rq with
 multiqueue
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To: Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Ming Lei <ming.lei@canonical.com>, Fam Zheng <famz@redhat.com>



On 21/05/2016 01:40, Stefan Hajnoczi wrote:
>      while (req) {
>          qemu_put_sbyte(f, 1);

Could you just put an extra 32-bit queue id here if num_queues > 1?  A
guest with num_queues > 1 cannot be started on pre-2.7 QEMU, so you can
change the migration format (if virtio were using vmstate, it would be a
VMSTATE_UINT32_TEST).

Thanks,

Paolo

>          qemu_put_virtqueue_element(f, &req->elem);
>          req = req->next;
> +        s->num_rq++;
>      }
>      qemu_put_sbyte(f, 0);
> +
> +    /* In order to distinguish virtio-blk subsections from the generic virtio
> +     * device subsections that follow we emit a terminating byte.  Old versions
> +     * of QEMU don't expect the terminating byte so, for compatibility, only
> +     * write it when virtio-blk subsections are needed.
> +     */
> +    if (virtio_blk_mq_rq_indices_needed(s)) {
> +        uint32_t i;
> +
> +        s->mq_rq_indices = g_new(uint32_t, s->num_rq);
> +        req = s->rq;
> +        for (i = 0; i < s->num_rq; i++) {
> +            s->mq_rq_indices[i] = virtio_get_queue_index(req->vq);
> +            req = req->next;
> +        }
> +
> +        vmstate_save_state(f, &virtio_blk_vmstate, s, NULL);
> +        qemu_put_ubyte(f, ~QEMU_VM_SUBSECTION);
> +
> +        g_free(s->mq_rq_indices);
> +        s->mq_rq_indices = NULL;