Re: [Xen-devel] [PATCH 09/10] tools/x86emul: Advertise more CPUID features for testing purposes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.17 at 15:37, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 27/03/17 13:13, Jan Beulich wrote:
>>>>> On 27.03.17 at 13:20, <george.dunlap@xxxxxxxxxx> wrote:
>>> On 27/03/17 10:56, Andrew Cooper wrote:
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>>> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>>> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
>>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>> ---
>>>>  tools/tests/x86_emulator/x86_emulate.c | 41 
> ++++++++++++++++++++++++----------
>>>>  1 file changed, 29 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/tools/tests/x86_emulator/x86_emulate.c 
>>> b/tools/tests/x86_emulator/x86_emulate.c
>>>> index cea0595..2c49954 100644
>>>> --- a/tools/tests/x86_emulator/x86_emulate.c
>>>> +++ b/tools/tests/x86_emulator/x86_emulate.c
>>>> @@ -73,20 +73,37 @@ int emul_test_cpuid(
>>>>           : "a" (leaf), "c" (subleaf));
>>>>  Oh, s
>>>>      /*
>>>> -     * The emulator doesn't itself use MOVBE, so we can always run the
>>>> -     * respective tests.
>>>> +     * Some instructions and features can be emulated without specific
>>>> +     * hardware support.  These features are unconditionally reported 
>>>> here,
>>>> +     * for testing and fuzzing-coverage purposes.
>>> But similarly to my question in patch 10 -- is there any chance that the
>>> emulator will ever be called with a cpuid callback that returns 'false"
>>> for these?  If so, isn't there therefore a chance that there will be
>>> some sort of bug which only triggers if these bits are set to 'false'?
>> I think I've suggested before that the cpuid hook should actually
>> return void, as it can't possibly fail (now that CPUID faulting is
>> being handled in generic code).
> 
> I've been considering this quite a lot recently.  One the one hand, the
> introspection hook for CPUID really ought to be using X86EMUL_RETRY.
> 
> On the other, we really are (ab)using the existing cpuid() hook for two
> different purposes.  There really is a conceptual difference between
> issuing a cpuid() call as part of emulating a CPUID instruction, and
> using it to find out whether other instructions are permitted.  The
> latter is synonymous to having or not having the requisite piece of
> silicon, and isn't something which can fail.

There may be a semantic difference, but a conceptual one? CPUID
insns can't fail either (with CPUID faulting out of the picture).

Jan


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