Re: [Qemu-devel] [PATCH 3/3] virtio: optimize virtio_access_is_big_endian() for little-endian targets

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

On Thu, 12 Nov 2015 19:08:59 +0100
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Mon, 09 Nov 2015 18:58:34 +0100
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > When adding cross-endian support, we introduced the TARGET_IS_BIENDIAN macro
> > and the virtio_access_is_big_endian() helper to have a branchless fast path
> > in the virtio memory accessors for targets that don't switch endian.
> > 
> > This was considered as a strong requirement at the time.
> > 
> > Now we have added a runtime check for virtio 1.0, which ruins the benefit
> > of the virtio_access_is_big_endian() helper for always little-endian targets.
> > 
> > With this patch, fixed little-endian targets stop checking for virtio 1.0,
> > since the result is little-endian in all cases. 
> 
> So always-LE gets optimized, while always-BE and bi-endian stay the same?
> 

Yes.

> (Is there a measurable impact?)
> 

I tried to measure using iperf between host and guest but I could not
find any significant change... do you think about another test I could
try ?

> > The helper also gets renamed
> > so it is clear it is optimized for fast paths.
> 
> Even if it isn't actually 'fast' on anything other than fixed-LE?

Yes this is definitely a fixed-LE only optimization... should I drop the name
change and add a comment instead ?

> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  include/hw/virtio/virtio-access.h |   43 ++++++++++++++++++++-----------------
> >  1 file changed, 23 insertions(+), 20 deletions(-)