Anthony Liguori wrote:
In theory vringfd will get us zero copy from guest sendfile out to
external machines. For anything else we're doing a copy anyway, so
avoiding copying has no great benefit.
There's nothing that prevents zero-copy to be implemented for tun
without vringfd. In fact, I seem to recall that your earlier patches
implemented zero-copy :-)
I like the vringfd model and I think it's a good way to move forward.
My concern is that it introduces an extra syscall in the TX path.
Right now, we do a single write call whereas with vringfd we need to
insert the TX packet into the queue, do a notify, and then wait for
indication that the TX has succeeded.
I know we'll win with TSO but we don't need vringfd for TSO. The
jury's still out IMHO as to whether we should do vringfd or just try
to merge TSO tun patches.
tun+tso still doesn't give you zerocopy (unless you change it to use
aio, which re-introduces the syscall).
btw, the two vringfd syscalls are amortized over a potentially large
number of packets, whereas the single tun syscall is per-packet.
(note: we can get rid of the two syscalls as well by having each side
opportunistically pick up ring entries, like Xen does)
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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