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Date: Tue, 24 Nov 2009 08:36:32 -0600 From: Anthony Liguori <anthony@...emonkey.ws> To: "Michael S. Tsirkin" <mst@...hat.com> CC: Rusty Russell <rusty@...tcorp.com.au>, Shirley Ma <mashirle@...ibm.com>, Eric Dumazet <eric.dumazet@...il.com>, Avi Kivity <avi@...hat.com>, netdev@...r.kernel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Hollis Blanchard <hollisb@...ibm.com> Subject: Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and big packets in virtio_net Michael S. Tsirkin wrote: > On Tue, Nov 24, 2009 at 08:54:23AM +1030, Rusty Russell wrote: > >> On Tue, 24 Nov 2009 02:37:01 am Shirley Ma wrote: >> >>>>> + skb = (struct sk_buff *)buf; >>>>> >>>> This cast is unnecessary, but a comment would be nice: >>>> >>> Without this cast there is a compile warning. >>> >> Hi Shirley, >> >> Looks like buf is a void *, so no cast should be necessary. But I could >> be reading the patch wrong. >> >> >>>> However, I question whether making it 16 byte is the right thing: the >>>> ethernet header is 14 bytes long, so don't we want 8 bytes of padding? >>>> >>> Because in QEMU it requires 10 bytes header in a separately, so one page >>> is used to share between virtio_net_hdr header which is 10 bytes head >>> and rest of data. So I put 6 bytes offset here between two buffers. I >>> didn't look at the reason why a seperate buf is used for virtio_net_hdr >>> in QEMU. >>> >> It's a qemu bug. It insists the header be an element in the scatterlist by >> itself. Unfortunately we have to accommodate it. >> > > We do? Let's just fix this? > So does lguest. It's been that way since the beginning. Fixing this would result in breaking older guests. We really need to introduce a feature bit if we want to change this. Regards, Anthony Liguori -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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