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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 25 Nov 2009 11:15:30 +0200

On Wed, Nov 25, 2009 at 10:42:06AM +1030, Rusty Russell wrote:
> 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...

Hmm, is it really worth it to save a header copy if it's linear?  We are
going to access it anyway, and it fits into one cacheline nicely.  On
the other hand we have more code making life harder for compiler and
processor.

> Thanks!
> Rusty.