Re: [PATCH 4/4] tools/tests/x86_emulator: disable xmm* tests on clang

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.02.2026 10:34, Edwin Torok wrote:
>> On 23 Feb 2026, at 16:10, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 23.02.2026 11:04, Edwin Török wrote:
>>> clang-21 doesn't support `-ffixed-xmm0`, so `%xmm0` won't have the
>>> expected value.
>>> Disable these tests on clang.
>>
>> I don't think that's what we want, and not only because of the clutter the
>> various #ifdef cause. We want to be able to run as many of the tests as
>> possible, so the first goal should be to find some alternative mechanism to
>> achieve the same effect. A global register variable comes to mind as a
>> possible option.
>>
>> Further, how did you arrive at which tests need suppressing?
> 
> I used gdb to look at the 2 memory areas, and noticed that the XMM region was 
> different between emulated and actual when built with clang.
> Then I noticed the build failures due to the lack of fixed-xmm0.
> Then I added the ifdefs one by one as I ran the test until the whole test 
> program passed without failure.
> I tried adding some ‘pxor xmm0, xmm0’ into the cpu_has_sse2 sections, but 
> that didn’t really work either.
> 
> Although I may have been misled by the overlapping region, see below.
> 
>> I don't think
>> we rely on an "expected value" anywhere. I don't even recall us passing
>> -ffixed-xmm0 when compiling test_x86_emulate.c.
> 
> Yes, I’m surprised it works with GCC. But maybe only because the emulator 
> overwrites the actual FXSAVE area corresponding to XMM.
> XMM0 begins at offset 160, and 0x100 - 0x80 = 128.
> AFAICT the actual execution stores its result at [0x80, 0x80+0x200), and the 
> emulator stores its result into [0x100, 0x100+0x200).
> So the emulator will overwrite some of the values from the actual run. 
> 
> This only works if the end of the FXSAVE area looks like its beginning (i.e. 
> if FCW/FSW/etc. happens to match MM6/etc.)

Are you possibly overlooking the fact that res[] is an array of unsigned int 
elements,
i.e. the offsets used in source code all need to be multiplied by 4 to give 
offsets in
memory?

Jan