[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 3/3] virtio: optimize virtio_access_is_big_endia
From: |
Greg Kurz |
Subject: |
Re: [Qemu-devel] [PATCH 3/3] virtio: optimize virtio_access_is_big_endian() for little-endian targets |
Date: |
Fri, 13 Nov 2015 10:28:31 +0100 |
On Thu, 12 Nov 2015 19:08:59 +0100
Cornelia Huck <address@hidden> wrote:
> On Mon, 09 Nov 2015 18:58:34 +0100
> Greg Kurz <address@hidden> 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 <address@hidden>
> > ---
> > include/hw/virtio/virtio-access.h | 43
> > ++++++++++++++++++++-----------------
> > 1 file changed, 23 insertions(+), 20 deletions(-)