netdev - Re: [PATCH 2/2] vhost_net: a kernel-level virtio server

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Date:	Thu, 13 Aug 2009 15:38:43 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	virtualization@...ts.linux-foundation.org,
	"Ira W. Snyder" <iws@...o.caltech.edu>, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vhost_net: a kernel-level virtio server

On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> On Wed, Aug 12, 2009 at 07:59:47PM +0200, Arnd Bergmann wrote:
> > The trick is to swap the virtqueues instead. virtio-net is actually
> > mostly symmetric in just the same way that the physical wires on a
> > twisted pair ethernet are symmetric (I like how that analogy fits).
> 
> You need to really squint hard for it to look symmetric.
> 
> For example, for RX, virtio allocates an skb, puts a descriptor on a
> ring and waits for host to fill it in. Host system can not do the same:
> guest does not have access to host memory.
> 
> You can do a copy in transport to hide this fact, but it will kill
> performance.

Yes, that is what I was suggesting all along. The actual copy operation
has to be done by the host transport, which is obviously different from
the guest transport that just calls the host using vring_kick().

Right now, the number of copy operations in your code is the same.
You are doing the copy a little bit later in skb_copy_datagram_iovec(),
which is indeed a very nice hack. Changing to a virtqueue based method
would imply that the host needs to add each skb_frag_t to its outbound
virtqueue, which then gets copied into the guests inbound virtqueue.

Unfortunately, this also implies that you could no longer simply use the
packet socket interface as you do currently, as I realized only now.
This obviously has a significant impact on your user space interface.

	Arnd <><
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