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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 01 Jun 2015 09:23:28 +0200
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Subject: Re: [Qemu-devel] [PATCH v2] virtio: make features 64bit wide
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, "open
	list:virtio-blk" <qemu-block@nongnu.org>, qemu-devel@nongnu.org, Amit Shah <amit.shah@redhat.com>, "Aneesh
	Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>, Cornelia Huck <cornelia.huck@de.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>

On Fr, 2015-05-29 at 16:53 +0200, Michael S. Tsirkin wrote:
> On Fri, May 29, 2015 at 09:51:20AM +0200, Gerd Hoffmann wrote:
> > Make features 64bit wide everywhere.  Exception: command line flags
> > remain 32bit and are copyed into the lower 32 host_features at
> > initialization time.
> > 
> > On migration a full 64bit guest_features field is sent if one of the
> > high bits is set, additionally to the lower 32bit guest_features field
> > which must stay for compatibility reasons.  That way we send the lower
> > 32 feature bits twice, but that way the code is simpler because we don't
> > have to split and compose the 64bit features into two 32bit fields.
> > 
> > This depends on "move host_features" patch by cornelia.
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> 
> Thanks, this is very close to what I had in mind.
> Question: why do we need the feature_flags field?
> What's wrong with setting bits in host_features directly?

DEFINE_PROP_BIT works on uint32_t.

Alternative approach would be to introduce a DEFINE_PROP_BIT64 and use
that for DEFINE_VIRTIO_COMMON_FEATURES.

cheers,
  Gerd