Paolo Bonzini <pbonz...@redhat.com> writes:
> From: Artyom Kunakovsky <artyomkunakov...@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 <artyomkunakov...@gmail.com>
> Message-ID: <20240523051118.29367-1-artyomkunakov...@gmail.com>
> [rewrite commit message, remove from configure. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonz...@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
