Re: [Xen-devel] [PATCH v3 09/11] fuzz/x86emul: update fuzzer

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.02.17 at 18:12, <wei.liu2@xxxxxxxxxx> wrote:
> On Thu, Feb 02, 2017 at 10:01:46AM -0700, Jan Beulich wrote:
>> >>> On 02.02.17 at 17:50, <wei.liu2@xxxxxxxxxx> wrote:
>> > On Thu, Feb 02, 2017 at 05:20:56AM -0700, Jan Beulich wrote:
>> >> >>> On 01.02.17 at 13:02, <wei.liu2@xxxxxxxxxx> wrote:
>> >> > +static int _fuzz_read_msr(
>> >> > +    unsigned int reg,
>> >> > +    uint64_t *val,
>> >> > +    struct x86_emulate_ctxt *ctxt)
>> >> > +{
>> >> > +    unsigned int idx;
>> >> > +
>> >> > +    switch ( reg )
>> >> > +    {
>> >> > +    case MSR_TSC_AUX:
>> >> > +    case MSR_IA32_TSC:
>> >> > +        return data_read("read_msr", val, sizeof(*val));
>> >> 
>> >> Strictly speaking the above applies to TSC_AUX too. And TSC should
>> >> return monotonically increasing values. I don't think though that
>> >> producing random output here matters right now. A comment may
>> >> be worthwhile.
>> >> 
>> > 
>> > Right, I will add the following:
>> > 
>> >         /*
>> >          * TSC should return monotonically increasing values, but
>> >          * returning random values is fine in fuzzer.
>> >          */
>> 
>> What about TSC_AUX then?
>> 
> 
> What model would you like it to follow? I suppose returning random value
> is also fine? I.e. I should just add TSC_AUX to the comment as well.

That's one option. Even better would be to consistently return
the same (possibly random) value. But the value itself isn't being
looked at by the emulator, and iirc the MSR also isn't ever being
read multiple times, so it really doesn't matter as long as the
caller(s) of x86_emulate() do(es)n't care.

Jan


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