Re: [Qemu-devel] [PATCH v8 00/20] virtio endian-ambivalent target

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Wed, 18 Jun 2014 16:42:04 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Wed, Jun 18, 2014 at 01:53:15PM +0100, Peter Maydell wrote:
> > On 18 June 2014 11:38, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > What bothers me is that real hardware can't do this.
> > 
> > Real hardware doesn't have "endianness matches guest CPU endianness"
> > semantics, which is what the virtio spec mandates...
> 
> So it was buggy. We never thought anyone would do a cross endian CPU :(.
> We are fixing it in 1.0.
> 

virtio isn't the only victim... we also have vga. The problem can pop up
anywhere you rely on TARGET_WORDS_BIGENDIAN.

> > > Given that VIRTIO
> > > 1.0 is always little-endian I guess this is just a temporary hack for
> > > ppc little-endian.  Would be nice to add a comment so it's clear why
> > > this approach is being taken instead of a cleaner solution.
> > 
> > Also for ARM big-endian, and indeed for any CPU with runtime
> > configurable endianness that wants to use the kernel virtio
> > drivers that exist in the real world rather than the theoretical
> > future ones that might some day be written for the 1.0 virtio
> > spec...
> > 
> > thanks
> > -- PMM
> 
> That's not a theoretical future.
> Spec will almost certainly be frozen two weeks from now.
> So it is almost certain that drivers will be there in 3.17. 
> 

I don't want argue on the dates but I doubt that all legacy users
will switch to 1.0 as soon as it shows up... a transition period
may be needed.

> Existing distros can then simply backport the
> drivers - same as they would with any other new hardware.
> 

Are you saying that upstream QEMU should not to support the
transition between legacy and 1.0 at all ?

Cheers.

-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.