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

[Richard Henderson <rth@twiddle.net>]

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 <rth@twiddle.net>
>> ---
>>  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~