Re: ia-32/ia-64 fxsave64 instruction behavior when saving mmx

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Robert.

Top-posting is difficult to read on technical lists,
it's better to reply inline.

Cc'ing the X86 FPU maintainers:

./scripts/get_maintainer.pl -f target/i386/fpu_helper.c
Paolo Bonzini <pbonzini@redhat.com> (maintainer:X86 TCG CPUs)
Richard Henderson <rth@twiddle.net> (maintainer:X86 TCG CPUs)
Eduardo Habkost <ehabkost@redhat.com> (maintainer:X86 TCG CPUs)

On 6/1/20 1:22 AM, Robert Henry wrote:
> Here's additional information.
> 
> All of the remill tests of the legacy MMX instructions fail. These
> instructions work on 64-bit registers aliased with the lower 64-bits of
> the x87 fp80 registers.  The tests fail because remill expects the
> fxsave64 instruction to deliver 16 bits of 1's (infinity or nan prefix)
> in the fp80 exponent, eg bits 79:64.  Metal does this, but QEMU does not.

Metal is what matters, QEMU should emulate it when possible.

> 
> Reading of Intel Software development manual, table 3.44
> (https://www.felixcloutier.com/x86/FXSAVE.html#tbl-3-44) says these 16
> bits are reserved, but another version of the manual
> (http://math-atlas.sourceforge.net/devel/arch/ia32_arch.pdf) section
> 9.6.2 "Transitions between x87 fpu and mmx code" says a write to an MMX
> register sets those 16 bits to all 1s.

You are [1] here answering [2] you asked below.

> 
> In digging through the code for the implementation of the SSE/mmx
> instruction pavgb I see a nice clean implementation in the SSE_HELPER_B
> macro which takes a MMXREG which is an MMREG_UNION which does not
> provide, to the extent that I can figure this out, a handle to bits
> 79:64 of the aliased-with x87 register.
> 
> I find it hard to believe that an apparent bug like this has been here
> "forever". Am I missing something?

Likely the developer who implemented this code didn't have all the
information you found, nor the test-suite, and eventually not even the
hardware to compare.

Since you have a good understanding of Intel FPU and hardware to
compare, do you mind sending a patch to have QEMU emulate the correct
hardware behavior?

If possible add a test case to tests/tcg/i386/test-i386.c (see
test_fxsave there).

> 
> Robert Henry
> ------------------------------------------------------------------------
> *From:* Robert Henry
> *Sent:* Friday, May 29, 2020 10:38 AM
> *To:* qemu-devel@nongnu.org <qemu-devel@nongnu.org>
> *Subject:* ia-32/ia-64 fxsave64 instruction behavior when saving mmx
>  
> Background: The ia-32/ia-64 fxsave64 instruction saves fp80 or legacy
> SSE mmx registers. The mmx registers are saved as if they were fp80
> values. The lower 64 bits of the constructed fp80 value is the mmx
> register.  The upper 16 bits of the constructed fp80 value are reserved;
> see the last row of table 3-44
> of https://www.felixcloutier.com/x86/fxsave#tbl-3-44
> 
> The Intel core i9-9980XE Skylake metal I have puts 0xffff into these
> reserved 16 bits when saving MMX.
> 
> QEMU appears to put 0's there.
> 
> Does anybody have insight as to what "reserved" really means, or must
> be, in this case?

You self-answered to this [2] in [1] earlier.

> I take the verb "reserved" to mean something other
> than "undefined".
> 
> I came across this issue when running the remill instruction test
> engine.  See my
> issue https://github.com/lifting-bits/remill/issues/423 For better or
> worse, remill assumes that those bits are 0xffff, not 0x0000
> 

Regards,

Phil.