Re: [PATCH 6/6] meson: require compiler support for chosen x86-64 instructions

[Richard Henderson <richard.henderson@linaro.org>]

On 6/20/24 08:01, Daniel P. Berrangé wrote:
> On Thu, Jun 20, 2024 at 03:02:54PM +0200, Paolo Bonzini wrote:
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   meson.build | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/meson.build b/meson.build
>> index 54e6b09f4fb..c5360fbd299 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -2863,6 +2863,7 @@ have_cpuid_h = cc.links('''
>>   config_host_data.set('CONFIG_CPUID_H', have_cpuid_h)
>>   
>>   config_host_data.set('CONFIG_AVX2_OPT', get_option('avx2') \
>> +  .enable_auto_if(get_option('x86_version') >= '3') \
>>     .require(have_cpuid_h, error_message: 'cpuid.h not available, cannot enable AVX2') \
>>     .require(cc.links('''
>>       #include <cpuid.h>
>> @@ -2875,6 +2876,7 @@ config_host_data.set('CONFIG_AVX2_OPT', get_option('avx2') \
>>     '''), error_message: 'AVX2 not available').allowed())
>>   
>>   config_host_data.set('CONFIG_AVX512BW_OPT', get_option('avx512bw') \
>> +  .enable_auto_if(get_option('x86_version') >= '4') \
>>     .require(have_cpuid_h, error_message: 'cpuid.h not available, cannot enable AVX512BW') \
>>     .require(cc.links('''
>>       #include <cpuid.h>
> 
> I'm not sure this makes sense. The CONFIG_AVX* options are used only
> to validate whether the toolchain has support for this. The QEMU
> code then has a runtime, so it automagically uses AVX2/AVX512
> if-and-only-if running on a suitably new CPU.  IOW, we want this
> enabled always when the toolchain supports it, regardless of what
> x86_version is set.

Indeed.  To me this means they should not be configure options at all.
We should simply detect compiler support, end of.


r~