netdev - Re: [RFC][PATCH v3 1/3] A device for zero-copy based on KVM virtio-net.

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Message-Id: <201004142239.50299.arnd@arndb.de>
Date:	Wed, 14 Apr 2010 22:39:49 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	xiaohui.xin@...el.com, netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, davem@...emloft.net,
	jdike@...ux.intel.com
Subject: Re: [RFC][PATCH v3 1/3] A device for zero-copy based on KVM virtio-net.

On Wednesday 14 April 2010 22:31:42 Michael S. Tsirkin wrote:
> On Wed, Apr 14, 2010 at 06:35:57PM +0200, Arnd Bergmann wrote:
> > On Wednesday 14 April 2010, Michael S. Tsirkin wrote:
> > > > > 
> > > > > qemu needs the ability to inject raw packets into device
> > > > > from userspace, bypassing vhost/virtio (for live migration).
> > > > 
> > > > Ok, but since there is only a write callback and no read, it won't
> > > > actually be able to do this with the current code, right?
> > > 
> > > I think it'll work as is, with vhost qemu only ever writes,
> > > never reads from device. We'll also never need GSO etc
> > > which is a large part of what tap does (and macvtap will
> > > have to do).
> > 
> > Ah, I see. I didn't realize that qemu needs to write to the
> > device even if vhost is used. But for the case of migration to
> > another machine without vhost, wouldn't qemu also need to read?
> 
> Not that I know. Why?

Well, if the guest not only wants to send data but also receive
frames coming from other machines, they need to get from the kernel
into qemu, and the only way I can see for doing that is to read
from this device if there is no vhost support around on the new
machine.

Maybe we're talking about different things here.

	Arnd
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