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    SubjectRe: [PATCH RFC 3/5] tun: vringfd receive support.
    Rusty Russell wrote:
    > On Wednesday 09 April 2008 05:49:15 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.
    >
    > I started modifying tun to do this directly, but it ended up with a whole heap
    > of code just for the rings, and a lot of current code (eg. read, write, poll)
    > ended up inside an 'if (tun->rings) ... else {'. Having a natural poll()
    > interface for the rings made more sense, so being their own fds fell out
    > naturally.
    Hmm, the version that I sent you awhile ago (remember I sent you an attachment
    with prototype of the new tun driver and user space code) was not that bad in
    that area. It mean it did not touch existing read()/write() path. The
    difference was that it allocated the rings and the data buffer in the kernel
    and mapped into the user-space. Which is not what you guys need but that's a
    separate thing.

    The fd thing could be an issue. As I mentioned the example would be a VPN
    server (OpenVPN, etc) with a bunch of client connection (typically tun per
    connection).

    > I decided to float this version because it does minimal damage to tun, and I
    > know that other people have wanted rings before: I'd like to know if this is
    > likely to be generic enough for them.
    I see.

    I'll try to spend some time on this in a near future and take a crack at the
    version with the TUN specific rings. Although I said that many times now and
    it may not happen in the near enough future :). In the mean time if your
    current version helps you guys a lot I do not mind us putting it in. We can
    always add another mode or something that uses internal rings and gradually
    obsolete old read()/write() and generic rings.

    Max


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