Re: [Xen-devel] [PATCH v2] x86emul: don't read mask register on AVX512F-incapable platforms

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/03/2019 09:36, Jan Beulich wrote:
>>>> On 29.03.19 at 10:19, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 29/03/2019 07:42, Jan Beulich wrote:
>>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>>> @@ -3511,7 +3511,8 @@ x86_emulate(
>>>      }
>>>  
>>>      /* With a memory operand, fetch the mask register in use (if any). */
>>> -    if ( ea.type == OP_MEM && evex.opmsk )
>>> +    if ( ea.type == OP_MEM && cpu_has_avx512f && evex.opmsk &&
>>> +         _get_fpu(fpu_type = X86EMUL_FPU_opmask, ctxt, ops) == 
>>> X86EMUL_OKAY )
>> The cpu_has_avx512f check is now redundant.  It is fully encapsulated by
>> _get_fpu() being happy with X86EMUL_FPU_opmask.
> Well, that'll end up being inconsistent with what we do elsewhere:
> If we did as you say, host_and_vcpu_must_have(avx512f) could
> (and for consistency then should) all become just
> vcpu_must_have(avx512f). Similarly for AVX.

That case isn't the same.

For kmov, we don't care about the instruction group per say.  We care
that kmask xsave state is active and usable.

If you recall, the reason why you chose not to merge the host_and_vcpu
and vcpu predicates when I queried this on initial review was for the
theoretical case of the guest being offered features not present in
hardware, and having the emulator fill in the gaps.  (Also, the code may
have pre-dated {pv,hvm}_cpuid() handing back properly audited content,
which is something that has definitely been fixed now.)

Given many years retrospect on the matter, I'm not actually sure how
much of a useful usecase this is.  Obviously, there are some
cross-vendor applicabilities, but these only extend to individual
instructions whose behaviour can be fully replaced in other ways (i.e.
not for instructions which we decode and replay).

I don't see us ever gaining support for using instructions in cases
where the relevant xstate isn't available in hardware.

>
> I'd like to put up the other option then: Rather than using
> _get_fpu() (and in particular the read_xcr() and read_cr() hooks)
> we could read the real XCR0 here. After all we issue the KMOV not
> because the guest has specified it, but because we need the value
> of the register for correct fault suppression emulation.

True, and that would be rather smaller and less invasive than
deliberately squashing the other side effects of get_fpu()

~Andrew

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