netdev - Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus

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Date:	Thu, 20 Aug 2009 10:57:43 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Hollis Blanchard <hollisb@...ibm.com>
Cc:	Avi Kivity <avi@...hat.com>,
	"Ira W. Snyder" <iws@...o.caltech.edu>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Gregory Haskins <gregory.haskins@...il.com>,
	kvm@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net,
	Anthony Liguori <anthony@...emonkey.ws>,
	Ingo Molnar <mingo@...e.hu>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus 
	model for vbus_driver objects

On Wed, Aug 19, 2009 at 10:05 PM, Hollis Blanchard<hollisb@...ibm.com> wrote:
> On Wed, 2009-08-19 at 19:38 +0300, Avi Kivity wrote:
>> On 08/19/2009 07:29 PM, Ira W. Snyder wrote:
>> >>> That said, I'm not sure how qemu-system-ppc running on x86 could
>> >>> possibly communicate using virtio-net. This would mean the guest is an
>> >>> emulated big-endian PPC, while the host is a little-endian x86. I
>> >>> haven't actually tested this situation, so perhaps I am wrong.

Cross-platform virtio works when endianness is known in advance.  For
a hypervisor and a guest:
1. virtio-pci I/O registers use PCI endianness
2. vring uses guest endianness (hypervisor must byteswap)
3. guest memory buffers use guest endianness (hypervisor must byteswap)

I know of no existing way when endianness is not known in advance.
Perhaps a transport bit could be added to mark the endianness of the
guest/driver side.  This can be negotiated because virtio-pci has a
known endianness.  After negotiation, the host knows whether or not
byteswapping is necessary for structures in guest memory.

Stefan
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