Re: [PATCH v2] meson.build: add -mcx16 flag for x86_64 host

[Paolo Bonzini <pbonzini@redhat.com>]

On 5/23/24 10:45, Daniel P. Berrangé wrote:
> On Thu, May 23, 2024 at 08:11:18AM +0300, Artyom Kunakovsky wrote:
>> Fix linker error if the project was configured by the './configure --cpu=unknown --target-list=riscv64-softmmu' command
> 
> As with v1, why are you intentionally passing a bogus CPU target
> name to the --cpu arg ?  QEMU already correctly sets '-mcx16' if
> you omit --cpu, or pass a correct "x86_64" target name to --cpu.

The patch has a point though, in that right above we have another test
to add -march=i486.  It's just that we do that one conditionally,
because most of the time the compiler will already apply the less-
restrictive -march=i686.

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.  And -mcx16 is not applied to cross-compiled
x86_64 code too, so why is it even in configure.

This is not to say that passing --cpu=unknown is a good idea; the
reason that Artyom gives is not really compelling.  But I think
I am going to apply it as a cleanup together with the matching
change to configure:

--------------- 8< -----------------
From: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
Subject: [PATCH] configure: move -mcx16 flag out of CPU_CFLAGS

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>

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


>>
>> Signed-off-by: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
>> ---
>>   meson.build | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/meson.build b/meson.build
>> index a9de71d450..e68fbfc662 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -336,6 +336,12 @@ if host_arch == 'i386' and not cc.links('''
>>     qemu_common_flags = ['-march=i486'] + qemu_common_flags
>>   endif
>>   
>> +
>> +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
>> -- 
>> 2.25.1
>>
>>
> 
> With regards,
> Daniel