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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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).

This isn't about failing so much as it is about reporting the presence /
absence of hardware features.  With this patch, cpuid unconditionally
advertises the presence of a number of features (MOVBE, rtm, ADCX/ADOX,
&c) because the emulation will work even if the features aren't actually
present in hardware.  I'm suggesting that we may want to make sure that
we test *both* the "feature is present" path, *and* the "feature is
missing" path.

 -George


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