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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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