Re: [Qemu-devel] [PATCH v2 10/27] tcg: Add atomic128 helpers

[Richard Henderson]

On 07/07/2016 08:00 PM, Emilio G. Cota wrote:
> On Fri, Jul 01, 2016 at 10:04:36 -0700, Richard Henderson wrote:
> > Force the use of cmpxchg16b on x86_64.
> >
> > Wikipedia suggests that only very old AMD64 (circa 2004) did not have
> > this instruction.  Further, it's required by Windows 8 so no new cpus
> > will ever omit it.
> >
> > If we truely care about these, then we could check this at startup time
> > and then avoid executing paths that use it.
> >
> > Signed-off-by: Richard Henderson <address@hidden>
> > ---
> >  configure             |  29 ++++++++++++-
> >  cputlb.c              |   6 +++
> >  include/qemu/int128.h |   6 +++
> >  softmmu_template.h    | 110 +++++++++++++++++++++++++++++++++++++-------------
> >  tcg/tcg.h             |  22 ++++++++++
> >  5 files changed, 144 insertions(+), 29 deletions(-)
> >
> > diff --git a/configure b/configure
> > index 59ea124..586abd6 100755
> > --- a/configure
> > +++ b/configure
> > @@ -1201,7 +1201,10 @@ case "$cpu" in
> >             cc_i386='$(CC) -m32'
> >             ;;
> >      x86_64)
> > -           CPU_CFLAGS="-m64"
> > +           # ??? Only extremely old AMD cpus do not have cmpxchg16b.
> > +           # If we truly care, we should simply detect this case at
> > +           # runtime and generate the fallback to serial emulation.
> > +           CPU_CFLAGS="-m64 -mcx16"
...
>    if compile_prog "" "" ; then
>      atomic128=yes
> +  elif compile_prog "-mcx16" "" ; then
> +    QEMU_CFLAGS="$QEMU_CFLAGS -mcx16"
> +    EXTRA_CFLAGS="$EXTRA_CFLAGS -mcx16"
> +    atomic128=yes

Your change doesn't change anything.  If the user gave -march=haswell, then 
cx16 is enabled and the first test succeeds.  If the user gave -march=k8, the 
initial test would fail, but the second test would explicitly add cx16, and the 
second test would succeed.

In all cases, cx16 gets enabled.


r~