linux-kernel - Re: [RFC v4 3/5] virtio_ring: add packed ring support

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Date:   Wed, 16 May 2018 20:51:43 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [RFC v4 3/5] virtio_ring: add packed ring support



On 2018年05月16日 20:39, Tiwei Bie wrote:
> On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote:
>> On 2018年05月16日 16:37, Tiwei Bie wrote:
> [...]
>>>    struct vring_virtqueue {
>>> @@ -116,6 +117,9 @@ struct vring_virtqueue {
>>>    			/* Last written value to driver->flags in
>>>    			 * guest byte order. */
>>>    			u16 event_flags_shadow;
>>> +
>>> +			/* ID allocation. */
>>> +			struct idr buffer_id;
>> I'm not sure idr is fit for the performance critical case here. Need to
>> measure its performance impact, especially if we have few unused slots.
> I'm also not sure.. But fortunately, it should be quite easy
> to replace it with something else without changing other code.
> If it will really hurt the performance, I'll change it.

We may want to do some benchmarking/profiling to see.

>
>>>    		};
>>>    	};
> [...]
>>> +static void detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
>>> +			      unsigned int id, void **ctx)
>>> +{
>>> +	struct vring_packed_desc *desc;
>>> +	unsigned int i, j;
>>> +
>>> +	/* Clear data ptr. */
>>> +	vq->desc_state[id].data = NULL;
>>> +
>>> +	i = head;
>>> +
>>> +	for (j = 0; j < vq->desc_state[id].num; j++) {
>>> +		desc = &vq->vring_packed.desc[i];
>>> +		vring_unmap_one_packed(vq, desc);
>> As mentioned in previous discussion, this probably won't work for the case
>> of out of order completion since it depends on the information in the
>> descriptor ring. We probably need to extend ctx to record such information.
> Above code doesn't depend on the information in the descriptor
> ring. The vq->desc_state[] is the extended ctx.
>
> Best regards,
> Tiwei Bie

Yes, but desc is a pointer to descriptor ring I think so 
vring_unmap_one_packed() still depends on the content of descriptor ring?

Thanks