Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and big packets in virtio_net

Rusty Russell
2009-11-25T00:12:05Z

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.