netdev - Re: updated: kvm networking todo wiki

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Date:	Thu, 30 May 2013 09:41:07 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jason Wang <jasowang@...hat.com>, herbert@...dor.hengli.com.au,
	kvm <kvm@...r.kernel.org>, qemu-devel <qemu-devel@...gnu.org>,
	netdev@...r.kernel.org,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Dmitry Fleytman <dmitry@...nix.com>
Subject: Re: updated: kvm networking todo wiki

"Michael S. Tsirkin" <mst@...hat.com> writes:

> On Thu, May 30, 2013 at 08:40:47AM -0500, Anthony Liguori wrote:
>> Stefan Hajnoczi <stefanha@...il.com> writes:
>> 
>> > On Thu, May 30, 2013 at 7:23 AM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>> >> Anthony Liguori <anthony@...emonkey.ws> writes:
>> >>> Rusty Russell <rusty@...tcorp.com.au> writes:
>> >>>> On Fri, May 24, 2013 at 08:47:58AM -0500, Anthony Liguori wrote:
>> >>>>> FWIW, I think what's more interesting is using vhost-net as a networking
>> >>>>> backend with virtio-net in QEMU being what's guest facing.
>> >>>>>
>> >>>>> In theory, this gives you the best of both worlds: QEMU acts as a first
>> >>>>> line of defense against a malicious guest while still getting the
>> >>>>> performance advantages of vhost-net (zero-copy).
>> >>>>>
>> >>>> It would be an interesting idea if we didn't already have the vhost
>> >>>> model where we don't need the userspace bounce.
>> >>>
>> >>> The model is very interesting for QEMU because then we can use vhost as
>> >>> a backend for other types of network adapters (like vmxnet3 or even
>> >>> e1000).
>> >>>
>> >>> It also helps for things like fault tolerance where we need to be able
>> >>> to control packet flow within QEMU.
>> >>
>> >> (CC's reduced, context added, Dmitry Fleytman added for vmxnet3 thoughts).
>> >>
>> >> Then I'm really confused as to what this would look like.  A zero copy
>> >> sendmsg?  We should be able to implement that today.
>> >>
>> >> On the receive side, what can we do better than readv?  If we need to
>> >> return to userspace to tell the guest that we've got a new packet, we
>> >> don't win on latency.  We might reduce syscall overhead with a
>> >> multi-dimensional readv to read multiple packets at once?
>> >
>> > Sounds like recvmmsg(2).
>> 
>> Could we map this to mergable rx buffers though?
>> 
>> Regards,
>> 
>> Anthony Liguori
>
> Yes because we don't have to complete buffers in order.

What I meant though was for GRO, we don't know how large the received
packet is going to be.  Mergable rx buffers lets us allocate a pool of
data for all incoming packets instead of allocating max packet size *
max packets.

recvmmsg expects an array of msghdrs and I presume each needs to be
given a fixed size.  So this seems incompatible with mergable rx
buffers.

Regards,

Anthony Liguori

>
>> >
>> > Stefan
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