Re: [Qemu-devel] [PATCH 01/15] s390x: fix virtio feature bitmap

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Apr 10, 2011 at 10:11:15PM +0200, Alexander Graf wrote:
> 
> On 10.04.2011, at 22:06, Aurelien Jarno wrote:
> 
> > On Sun, Apr 10, 2011 at 09:26:26PM +0200, Alexander Graf wrote:
> >> 
> >> On 10.04.2011, at 21:25, Aurelien Jarno wrote:
> >> 
> >>> On Mon, Apr 04, 2011 at 04:32:10PM +0200, Alexander Graf wrote:
> >>>> The feature bitmap in the s390 virtio machine is little endian. To
> >>>> address for that, we need to bswap the values after reading them out.
> >>>> 
> >>>> Signed-off-by: Alexander Graf <agraf@suse.de>
> >>>> ---
> >>>> hw/s390-virtio-bus.c |    4 ++--
> >>>> 1 files changed, 2 insertions(+), 2 deletions(-)
> >>>> 
> >>>> diff --git a/hw/s390-virtio-bus.c b/hw/s390-virtio-bus.c
> >>>> index 58af164..60e0135 100644
> >>>> --- a/hw/s390-virtio-bus.c
> >>>> +++ b/hw/s390-virtio-bus.c
> >>>> @@ -223,7 +223,7 @@ void s390_virtio_device_sync(VirtIOS390Device *dev)
> >>>>    cur_offs += num_vq * VIRTIO_VQCONFIG_LEN;
> >>>> 
> >>>>    /* Sync feature bitmap */
> >>>> -    stl_phys(cur_offs, dev->host_features);
> >>>> +    stl_phys(cur_offs, bswap32(dev->host_features));
> >>> 
> >>> Is bswap32 correct here for both big and little endian guests? I don't
> >>> really understand the reason why a bswap is needed here, especially
> >>> given that AFAIK this code was already used when using KVM.
> >> 
> >> This is target specific code. The s390-virtio-bus is s390 specific. And yes, the code was also broken with KVM. That's how I first found it actually.
> >> 
> > 
> > So in short, s390-virtio-bus is specific to big endian machines, but is
> > using little endian? That looks a bit weired. I guess it's probably to
> > late to change the specification anyway...
> 
> Yeah. In fact, it mixes big and little endian pieces. But it really is too late to change the (unwritten) spec, unfortunately.
> 
Ok, thanks for the explanations, I have applied it.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net