Re: [Qemu-devel] [PATCH v2 1/2] virtio-net: use the backend cross-endian capabilities

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

On Fri, 13 Nov 2015 15:46:06 +0100
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Fri, 13 Nov 2015 09:26:26 +0100
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > On Thu, 12 Nov 2015 18:52:55 +0100
> > Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> > 
> > > On Mon, 09 Nov 2015 18:41:33 +0100
> > > Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > > > +static bool virtio_net_needs_hdr_swap(VirtIONet *n)
> > > > +{
> > > > +    /* virtio_needs_swap() is constant for fixed endian targets: call it
> > > > +     * first to filter them out without penalty.
> > > 
> > > What do you mean with 'constant' here? It still needs to retrieve the
> > > feature bit from the device, no?
> > > 
> > 
> > Yes the comment is inaccurate... With the "virtio: cross-endian helpers fixes"
> > series, virtio_needs_swap() indeed resolves to { return 0; } for fixed little
> > endian targets but we still need to check the feature bit when the target is big
> > endian.
> > 
> > If I drop the call to virtio_needs_swap(), all targets will have the same
> > penalty. If I keep it, fixed little endian targets have no penalty but fixed
> > big endian targets get an extra check and bi-endian targets get two extra checks...
> > 
> > Not sure what to decide...
> 
> The impact probably isn't too large, but improving one configuration
> via penalizing others feels wrong.
> 
> Just check the bool value to make the decision?

Sounds fair. I'll drop the call to virtio_needs_swap() then.

Thanks.

--
Greg