netdev - Re: Using virtio as a physical (wire-level) transport

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Message-Id: <201008160949.41348.rusty@rustcorp.com.au>
Date:	Mon, 16 Aug 2010 09:49:40 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	virtualization@...ts.linux-foundation.org
Cc:	Alexander Graf <agraf@...e.de>,
	"Ira W. Snyder" <iws@...o.caltech.edu>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Zang Roy <r61911@...escale.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: Using virtio as a physical (wire-level) transport

On Sat, 14 Aug 2010 09:04:19 pm Alexander Graf wrote:
> 
> Am 06.08.2010 um 11:34 schrieb "Ira W. Snyder" <iws@...o.caltech.edu>:
> > This problem is not limited to my new use of virtio. Virtio is
> > completely useless in a relatively common virtualization scenario:
> > x86 host with qemu-ppc guest. Or any other big endian guest system.
> 
> This one actually works because we know that we're building for a BE guest.
> But I agree that it's a mess and clearly a very incorrect design decision.

Yes, since you need to know the guest's endian to virtualize it, the
correct interpretation of the virtio ring seemed the least problem.  Perhaps
I went overboard in simplification here, but it seemed pure legacy.

If we did a virtio2, as has been suggested, it would be possible to address
this.  You could of course do a hack where you detect the ring endianness
the first time they use it (based on avail.flags, avail.index and the
descriptor it would be quite reliable in practice).

Cheers,
Rusty.
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