Re: [virtio-dev] Re: [PATCH v11 6/6] virtio-balloon: VIRTIO_BALLOON_F_CMD_VQ

[Wei Wang]

On 06/21/2017 08:28 PM, Michael S. Tsirkin wrote:
> On Wed, Jun 21, 2017 at 11:28:00AM +0800, Wei Wang wrote:
> > On 06/21/2017 12:18 AM, Michael S. Tsirkin wrote:
> > > On Fri, Jun 09, 2017 at 06:41:41PM +0800, Wei Wang wrote:
> > > > -	if (!virtqueue_indirect_desc_table_add(vq, desc, num)) {
> > > > +	if (!virtqueue_indirect_desc_table_add(vq, desc, *num)) {
> > > >    		virtqueue_kick(vq);
> > > > -		wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> > > > -		vb->balloon_page_chunk.chunk_num = 0;
> > > > +		if (busy_wait)
> > > > +			while (!virtqueue_get_buf(vq, &len) &&
> > > > +			       !virtqueue_is_broken(vq))
> > > > +				cpu_relax();
> > > > +		else
> > > > +			wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> > > This is something I didn't previously notice.
> > > As you always keep a single buffer in flight, you do not
> > > really need indirect at all. Just add all descriptors
> > > in the ring directly, then kick.
> > >
> > > E.g.
> > > 	virtqueue_add_first
> > > 	virtqueue_add_next
> > > 	virtqueue_add_last
> > >
> > > ?
> > >
> > > You also want a flag to avoid allocations but there's no need to do it
> > > per descriptor, set it on vq.
> > Without using the indirect table, I'm thinking about changing to use
> > the standard sg (i.e. struct scatterlist), instead of vring_desc, so that
> > we don't need to modify or add any new functions of virtqueue_add().
> >
> > In this case, we will kmalloc an array of sgs in probe(), and we can add
> > the sgs one by one to the vq, which won't trigger the allocation of an
> > indirect table inside virtqueue_add(), and then kick when all are added.
> >
> > Best,
> > Wei
> And allocate headers too? This can work. API extensions aren't
> necessarily a bad idea though. The API I suggest above is preferable
> for the simple reason that it can work without INDIRECT flag
> support in hypervisor.

OK, probably we don't need to add a desc to the vq - we can just use
the vq's desc, like this:

int virtqueue_add_first(struct virtqueue *_vq,
                                     uint64_t addr,
                                     uint32_t len,
                                     bool in,
                                     unsigned int *idx) {

    ...
   uint16_t desc_flags = in ? VRING_DESC_F_NEXT | VRING_DESC_F_WRITE :
                                             VRING_DESC_F_NEXT;

    vq->vring.desc[vq->free_head].addr = addr;
    vq->vring.desc[vq->free_head].len = len;
    vq->vring.desc[vq->free_head].flags = cpu_to_virtio16(_vq->vdev, 
flags);
    /* return to the caller the desc id */
    *idx = vq->free_head;
    ...
}

int virtqueue_add_next(struct virtqueue *_vq,
                                     uint64_t addr,
                                     uint32_t len,
                                     bool in,
                                     bool end,
                                     unsigned int *idx) {
    ...
    vq->vring.desc[*idx].next = vq->free_head;
    vq->vring.desc[vq->free_head].addr = addr;
    ...
    if (end)
        remove the VRING_DESC_F_NEXT flag
}


What do you think? We can also combine the two functions into one.



Best,
Wei