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

[Anthony Liguori]

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
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