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

[Arseniy Krasnov]

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@xxxxxxxxxxxxxx>
>> Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
>> ---
>>  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.

Thanks, Arseniy

> 
>> -- 
>> 2.25.1
>