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

[Edwin Torok <edwin.torok@xxxxxxxxxx>]

> 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.)

If I move the regions, such that they are distinct, then this begins to fail 
with GCC too (perhaps due to the lack of fixed-xmm0, I haven’t tried).

Perhaps a better way to fix this would be to make the 2 regions distinct first, 
get it to work with GCC and then see what bugs remain on Clang.
I’ll try that approach, and see how far I get.

Best regards,
—Edwin

> We use that option when
> building various of the test blobs, iirc. And the comment ahead of the
> first use explains why we use the option there. (Later we also use
> -ffixed-ymm<N> and -ffixed-zmm<N>, btw.)
> 
> Jan