Re: [PATCH RFC 3/5] tun: vringfd receive support.

[Max Krasnyanskiy]

Dor Laor wrote:
> On Tue, 2008-04-08 at 12:49 -0700, Max Krasnyansky wrote:
> > Rusty Russell wrote:
> > > This patch modifies tun to allow a vringfd to specify the receive
> > > buffer.  Because we can't copy to userspace in bh context, we queue
> > > like normal then use the "pull" hook to actually do the copy.
> > >
> > > More thought needs to be put into the possible races with ring
> > > registration and a simultaneous close, for example (see FIXME).
> > >
> > > We use struct virtio_net_hdr prepended to packets in the ring to allow
> > > userspace to receive GSO packets in future (at the moment, the tun
> > > driver doesn't tell the stack it can handle them, so these cases are
> > > never taken).
> > In general the code looks good. The only thing I could not convince myself in
> > is whether having generic ring buffer makes sense or not.
> > At least the TUN driver would be more efficient if it had its own simple ring
> >  implementation. Less indirection, fewer callbacks, fewer if()s, etc. TUN
> > already has the file descriptor and having two additional fds for rx and tx
> > ring is a waste (think of a VPN server that has to have a bunch of TUN fds).
> > Also as I mentioned before Jamal and I wanted to expose some of the SKB fields
> > through TUN device. With the rx/tx rings the natural way of doing that would
> > be the ring descriptor itself. It can of course be done the same way we copy
> > proto info (PI) and GSO stuff before the packet but that means more
> > copy_to_user() calls and yet more checks.
> >
> > So. What am I missing ? Why do we need generic ring for the TUN ? I looked at
> > the lguest code a bit and it seems that we need a bunch of network specific
> > code anyway. The cool thing is that you can now mmap the rings into the guest
> >  directly but the same thing can be done with TUN specific rings.
>
> The idea was to use the same virtio ring that the guests use.
> The thing with TUN specific ring is that the guests are the one
> allocating the rings within their memory space and the opposite makes
> life very complex.

We can do the same thing with TUN rings. I mean have them allocated in the 
guest space. With that we'd still have all of the advantages that I listed 
above. ie We'd have ring descriptors that carry packet info, less indirection, 
etc.

Max


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