Re: [Xen-devel] [PATCH] x86emul: honor MXCSR.MM

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.10.16 at 15:26, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/10/16 13:57, Jan Beulich wrote:
>> Commit 6dc9ac9f52 ("x86emul: check alignment of SSE and AVX memory
>> operands") didn't consider a specific AMD mode: Mis-alignment #GP
>> faults can be masked on some of their hardware.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> This highlights that the following CPUID dependency change is also required
> 
> diff --git a/xen/tools/gen-cpuid.py b/xen/tools/gen-cpuid.py
> index 33e68eb..e803654 100755
> --- a/xen/tools/gen-cpuid.py
> +++ b/xen/tools/gen-cpuid.py
> @@ -185,8 +185,9 @@ def crunch_numbers(state):
>          # the first place.
>          APIC: [X2APIC],
>  
> -        # AMD built MMXExtentions and 3DNow as extentions to MMX.
> -        MMX: [MMXEXT, _3DNOW],
> +        # AMD built MMXExtentions, 3DNow and SSE Misalignment as
> extensions to
> +        # MMX.
> +        MMX: [MMXEXT, _3DNOW, MISALIGNSSE],
>  
>          # The FXSAVE/FXRSTOR instructions were introduced into hardware
> before
>          # SSE, which is why they behave differently based on
> %CR4.OSFXSAVE and

Hmm, for one this is an orthogonal change, so doesn't belong in this
patch. And then - why is this an extension to MMX (which doesn't
have any alignment requirements anyway) rather than SSE?

>> @@ -4675,7 +4679,13 @@ x86_emulate(
>>              ea.bytes = vex.pfx & VEX_PREFIX_DOUBLE_MASK ? 8 : 4;
>>          if ( ea.type == OP_MEM )
>>          {
>> -            generate_exception_if((b >= 0x28) &&
>> +            uint32_t mxcsr = 0;
>> +
>> +            if ( b < 0x28 )
>> +                mxcsr = MXCSR_MM;
>> +            else if ( vcpu_has_misalignsse() )
>> +                asm ( "stmxcsr %0" : "=m" (mxcsr) );
>> +            generate_exception_if(!(mxcsr & MXCSR_MM) &&
>>                                    !is_aligned(ea.mem.seg, ea.mem.off, 
>> ea.bytes,
>>                                                ctxt, ops),
>>                                    EXC_GP, 0);
>> @@ -4955,7 +4965,13 @@ x86_emulate(
>>          }
>>          if ( ea.type == OP_MEM )
>>          {
>> -            generate_exception_if((vex.pfx == vex_66) &&
>> +            uint32_t mxcsr = 0;
>> +
>> +            if ( vex.pfx != vex_66 )
>> +                mxcsr = MXCSR_MM;
>> +            else if ( vcpu_has_misalignsse() )
>> +                asm ( "stmxcsr %0" : "=m" (mxcsr) );
>> +            generate_exception_if(!(mxcsr & MXCSR_MM) &&
>>                                    !is_aligned(ea.mem.seg, ea.mem.off, 
>> ea.bytes,
>>                                                ctxt, ops),
>>                                    EXC_GP, 0);
> 
> According to the docs, we should also be possibly raising #AC here.

Well, you've said the same in a different context not so long ago,
and my answer is then also the same: As long as we don't do any
#AC generation, I see no reason why we would want to create an
exception here.

Jan


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