Re: [PULL 01/24] configure: move -mcx16 flag out of CPU_CFLAGS

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> From: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
>
> The point of CPU_CFLAGS is really just to select the appropriate multilib,
> for example for library linking tests, and -mcx16 is not needed for
> that purpose.
>
> Furthermore, if -mcx16 is part of QEMU's choice of a basic x86_64
> instruction set, it should be applied to cross-compiled x86_64 code too;
> it is plausible that tests/tcg would want to cover cmpxchg16b as well,
> for example.  In the end this makes just as much sense as a per sub-build
> tweak, so move the flag to meson.build and cross_cc_cflags_x86_64.
>
> This leaves out contrib/plugins, which would fail when attempting to use
> __sync_val_compare_and_swap_16 (note it does not do yet); while minor,
> this *is* a disadvantage of this change.  But building contrib/plugins
> with a Makefile instead of meson.build is something self-inflicted just
> for the sake of showing that it can be done, and if this kind of papercut
> started becoming a problem we could make the directory part of the meson
> build.  Until then, we can live with the limitation.
>
> Signed-off-by: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
> Message-ID: <20240523051118.29367-1-artyomkunakovsky@gmail.com>
> [rewrite commit message, remove from configure. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  configure   | 7 ++-----
>  meson.build | 7 +++++++
>  2 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/configure b/configure
> index 38ee2577013..4d01a42ba65 100755
> --- a/configure
> +++ b/configure
> @@ -512,10 +512,7 @@ case "$cpu" in
>      cpu="x86_64"
>      host_arch=x86_64
>      linux_arch=x86
> -    # ??? 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"
> +    CPU_CFLAGS="-m64"
>      ;;
>  esac
>  
> @@ -1203,7 +1200,7 @@ fi
>  : ${cross_cc_cflags_sparc64="-m64 -mcpu=ultrasparc"}
>  : ${cross_cc_sparc="$cross_cc_sparc64"}
>  : ${cross_cc_cflags_sparc="-m32 -mcpu=supersparc"}
> -: ${cross_cc_cflags_x86_64="-m64"}
> +: ${cross_cc_cflags_x86_64="-m64 -mcx16"}
>  
>  compute_target_variable() {
>    eval "$2="
> diff --git a/meson.build b/meson.build
> index a9de71d4506..7fd82b5f48c 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -336,6 +336,13 @@ if host_arch == 'i386' and not cc.links('''
>    qemu_common_flags = ['-march=i486'] + qemu_common_flags
>  endif
>  
> +# ??? 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.
> +if host_arch == 'x86_64'
> +  qemu_common_flags = ['-mcx16'] + qemu_common_flags
> +endif
> +
>  if get_option('prefer_static')
>    qemu_ldflags += get_option('b_pie') ? '-static-pie' : '-static'
>  endif

This breaks atomic detection resulting in:

#undef CONFIG_ATOMIC128
#undef CONFIG_ATOMIC128_OPT
#undef CONFIG_CMPXCHG128

which makes the TCG atomic handling code fallback to cpu_step_atomic,
killing performance.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro