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

[Wei Wang <wei.w.wang@intel.com>]

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

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