On Fri, Oct 04, 2024 at 03:01:23PM -0700, Pierrick Bouvier wrote:
> Alex discovered that CMPXCHG128 was not enabled when building for
> x86_64, resulting in slow execution for wide atomic instructions,
> creating a huge contention when combined with a high number of cpus
> (found while booting android aarch64 guest on x86_64 host).
>
> The problem is that even though we enable -mcx16 option for x86_64, this
> is not used when testing for CMPXCHG128. Thus, we silently turn it off.
>
> x86_64 is the only architecture adding machine flags for now, so the
> problem is limited to this host architecture.
>
> Meson compiler tests are supposed to be independent of environment flags
> (https://mesonbuild.com/Reference-manual_returned_compiler.html#returned-by).
> However, CFLAGS are used anyway, thus masking the problem when using
> something like CFLAGS='-march=native'. This is a meson bug and was reported:
> https://github.com/mesonbuild/meson/issues/13757
>
> Signed-off-by: Pierrick Bouvier <[email protected]>
> ---
> meson.build | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/meson.build b/meson.build
> index b18c2a54ab5..af2ce595dcc 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -2867,6 +2867,13 @@ if has_int128_type
> config_host_data.set('CONFIG_ATOMIC128_OPT', has_atomic128_opt)
>
> if not has_atomic128_opt
> +
> + host_flags = []
> + if host_arch == 'x86_64'
> + # for x86_64, x86_version must be >= 1, and we always enable
> cmpxchg16
> + # in this case.
> + host_flags += ['-mcx16']
> + endif
IMHO, we shouldn't be assuming that the earlier code passed '-mcx16',
as it requires that we keep 2 far away parts of meson.build in sync.
> config_host_data.set('CONFIG_CMPXCHG128', cc.links('''
> int main(void)
> {
> @@ -2874,7 +2881,8 @@ if has_int128_type
> __sync_val_compare_and_swap_16(&x, y, x);
> return 0;
> }
> - '''))
> + ''',
> + args: host_flags))
Just pass 'qemu_common_flags' here.
> endif
> endif
> endif
With regards,
Daniel
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