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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 27/03/17 14:03, Andrew Cooper wrote:
> On 27/03/17 13:56, George Dunlap 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).
>> 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.
> 
> I have some plans to make this happen, but it isn't easy with the
> existing infrastructure.  In the meantime, It is more important to get
> better coverage.

That sounds reasonable.

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>


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