netdev - Re: [PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add

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Message-ID: <877g03crwk.fsf@rustcorp.com.au>
Date:	Tue, 14 Oct 2014 12:51:47 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Cc:	netdev <netdev@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add_*()

"Michael S. Tsirkin" <mst@...hat.com> writes:
> On Fri, Sep 05, 2014 at 12:40:50PM +0200, Paolo Bonzini wrote:
>> Il 03/09/2014 06:29, Rusty Russell ha scritto:
>> > +	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
>> 
>> I think 2 is enough here.  That said...
>> 
>> >  	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
>> > -
>> >  	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
>> >  
>> >  	err = virtqueue_add_inbuf(rq->vq, rq->sg, 2, skb, gfp);
>> 
>> ... skb_to_sgvec will already make the sg well formed, so the
>> sg_init_table is _almost_ redundant; it is only there to remove
>> intermediate end marks.  The block layer takes care to remove
>> them, but skb_to_sgvec doesn't.

sg_init_table is still needed if CONFIG_DEBUG_SG, so I don't
think it's worth it.

Thanks,
Rusty.
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