Re: [PATCH for-8.1] tcg: Use HAVE_CMPXCHG128 instead of CONFIG_CMPXCHG128

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Richard,

On 13/7/23 22:23, Richard Henderson wrote:
> We adjust CONFIG_ATOMIC128 and CONFIG_CMPXCHG128 with
> CONFIG_ATOMIC128_OPT in atomic128.h.  It is difficult
> to tell when those changes have been applied with the
> ifdef we must use with CONFIG_CMPXCHG128.  So instead
> use HAVE_CMPXCHG128, which triggers -Werror-undef when
> the proper header has not been included.
> 
> Improves tcg_gen_atomic_cmpxchg_i128 for s390x host, which
> requires CONFIG_ATOMIC128_OPT.  Without this we fall back
> to EXCP_ATOMIC to single-step 128-bit atomics, which is
> slow enough to cause some tests to time out.
> 
> Reported-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> 
> Thomas, this issue does not quite match the one you bisected, but
> other than the cmpxchg, I don't see any see any qemu_{ld,st}_i128
> being used in BootLinuxS390X.test_s390_ccw_virtio_tcg.
> 
> As far as I can see, this wasn't broken by the addition of
> CONFIG_ATOMIC128_OPT, rather that fix didn't go far enough.
> 
> Anyway, test_s390_ccw_virtio_tcg now passes in 159s on our host.

IIUC:

If we have CONFIG_ATOMIC128, we use qatomic_cmpxchg__nocheck;
else if we have CONFIG_CMPXCHG128 we use __sync_val_compare_and_swap_16;
in both cases we set HAVE_CMPXCHG128;
otherwise we can not use atomic128 cmpxchg().

(I'm trying to figure why we need both CONFIGs).