[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <200911251042.06368.rusty@rustcorp.com.au>
Date: Wed, 25 Nov 2009 10:42:06 +1030
From: Rusty Russell <rusty@...tcorp.com.au>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: 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
On Tue, 24 Nov 2009 10:07:54 pm 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?
> All we have to do is replace memcpy with proper iovec walk, correct?
> Something like the followng (untested) patch? It's probably not too
> late to put this in the next qemu release...
You might want to implement a more generic helper which does:
/* Return pointer into iovec if we can, otherwise copy into buf */
void *pull_iovec(struct iovec *iov, int iovcnt, void *buf, size_t len)
{
unsigned int i;
void *p;
if (likely(iov_cnt && iov[0].iov_len >= len)) {
/* Nice contiguous chunk. */
void *p = iov[0].iov_base;
iov[i].iov_base += len;
iov[i].iov_len -= len;
return p;
}
p = buf;
for (i = 0; i < iov_cnt; i++) {
size_t this_len = min(len, iov[i].iov_len);
memcpy(p, iov[i].iov_base, this_len);
len -= this_len;
iov[i].iov_base += len;
iov[i].iov_len -= len;
if (len == 0)
return buf;
}
/* BTW, we screwed your iovec. */
return NULL;
}
Then use it in all the virtio drivers...
Thanks!
Rusty.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists