netdev - Re: [PATCH net-next v2 2/4] vsock/virtio: support to send non-linear skb

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Date: Wed, 19 Jul 2023 09:36:27 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, 
	oxffffaa@...il.com
Subject: Re: [PATCH net-next v2 2/4] vsock/virtio: support to send non-linear
 skb

On Wed, Jul 19, 2023 at 07:46:05AM +0300, Arseniy Krasnov wrote:
>
>
>On 18.07.2023 23:27, Michael S. Tsirkin wrote:
>> On Tue, Jul 18, 2023 at 09:02:35PM +0300, Arseniy Krasnov wrote:
>>> For non-linear skb use its pages from fragment array as buffers in
>>> virtio tx queue. These pages are already pinned by 'get_user_pages()'
>>> during such skb creation.
>>>
>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>>> Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>
>>> ---
>>>  net/vmw_vsock/virtio_transport.c | 40 +++++++++++++++++++++++++++-----
>>>  1 file changed, 34 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>>> index e95df847176b..6cbb45bb12d2 100644
>>> --- a/net/vmw_vsock/virtio_transport.c
>>> +++ b/net/vmw_vsock/virtio_transport.c
>>> @@ -100,7 +100,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>>  	vq = vsock->vqs[VSOCK_VQ_TX];
>>>
>>>  	for (;;) {
>>> -		struct scatterlist hdr, buf, *sgs[2];
>>> +		/* +1 is for packet header. */
>>> +		struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
>>> +		struct scatterlist bufs[MAX_SKB_FRAGS + 1];
>>>  		int ret, in_sg = 0, out_sg = 0;
>>>  		struct sk_buff *skb;
>>>  		bool reply;
>>> @@ -111,12 +113,38 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>>
>>>  		virtio_transport_deliver_tap_pkt(skb);
>>>  		reply = virtio_vsock_skb_reply(skb);
>>> +		sg_init_one(&bufs[out_sg], virtio_vsock_hdr(skb),
>>> +			    sizeof(*virtio_vsock_hdr(skb)));
>>> +		sgs[out_sg] = &bufs[out_sg];
>>> +		out_sg++;
>>> +
>>> +		if (!skb_is_nonlinear(skb)) {
>>> +			if (skb->len > 0) {
>>> +				sg_init_one(&bufs[out_sg], skb->data, skb->len);
>>> +				sgs[out_sg] = &bufs[out_sg];
>>> +				out_sg++;
>>> +			}
>>> +		} else {
>>> +			struct skb_shared_info *si;
>>> +			int i;
>>> +
>>> +			si = skb_shinfo(skb);
>>> +
>>> +			for (i = 0; i < si->nr_frags; i++) {
>>> +				skb_frag_t *skb_frag = &si->frags[i];
>>> +				void *va = page_to_virt(skb_frag->bv_page);
>>>
>>> -		sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
>>> -		sgs[out_sg++] = &hdr;
>>> -		if (skb->len > 0) {
>>> -			sg_init_one(&buf, skb->data, skb->len);
>>> -			sgs[out_sg++] = &buf;
>>> +				/* We will use 'page_to_virt()' for userspace page here,
>>
>> don't put comments after code they refer to, please?
>>
>>> +				 * because virtio layer will call 'virt_to_phys()' later
>>
>> it will but not always. sometimes it's the dma mapping layer.
>>
>>
>>> +				 * to fill buffer descriptor. We don't touch memory at
>>> +				 * "virtual" address of this page.
>>
>>
>> you need to stick "the" in a bunch of places above.
>
>Ok, I'll fix this comment!
>
>>
>>> +				 */
>>> +				sg_init_one(&bufs[out_sg],
>>> +					    va + skb_frag->bv_offset,
>>> +					    skb_frag->bv_len);
>>> +				sgs[out_sg] = &bufs[out_sg];
>>> +				out_sg++;
>>> +			}
>>>  		}
>>>
>>>  		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
>>
>>
>> There's a problem here: if there vq is small this will fail.
>> So you really should check free vq s/gs and switch to non-zcopy
>> if too small.
>
>Ok, so idea is that:
>
>if (out_sg > vq->num_free)
>    reorganise current skb for copy mode (e.g. 2 out_sg - header and data)
>    and try to add it to vq again.
>
>?
>
>@Stefano, I'll remove net-next tag (guess RFC is not required again, but not net-next
>anyway) as this change will require review. R-b I think should be also removed. All
>other patches in this set still unchanged.

It's still a new feature so we have net-next tree as the target, right?

I think we should keep net-next. Even if patches require to be
re-reviewed, net-next indicates the tree where we want these to be merge
and for new features is the right one.

Ack for not putting RFC again and for R-b removal for this patch.

Thanks,
Stefano