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

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

On 06/28/2017 11:01 PM, Michael S. Tsirkin wrote:
> On Thu, Jun 22, 2017 at 04:40:39PM +0800, Wei Wang wrote:
>> 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
>> }
>>
> Add I would say add-last.
>
>> What do you think? We can also combine the two functions into one.
>>
>>
>>
>> Best,
>> Wei
> With an enum? Yes that's also an option.
>

Thanks for the suggestion. I shifted it a little bit, please have a check
the latest v12 patches that I just sent out.

Best,
Wei

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