Re: [Xen-devel] [PATCH 5/5] xen: clean up VPF flags macros

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.09.15 at 10:15, <JGross@xxxxxxxx> wrote:
> On 09/28/2015 09:52 AM, Jan Beulich wrote:
>>>>> On 28.09.15 at 09:29, <JGross@xxxxxxxx> wrote:
>>> On 09/28/2015 08:22 AM, Jan Beulich wrote:
>>>>>>> On 28.09.15 at 07:23, <JGross@xxxxxxxx> wrote:
>>>>> On 09/25/2015 05:42 PM, Jan Beulich wrote:
>>>>>>>>> On 25.09.15 at 13:54, <JGross@xxxxxxxx> wrote:
>>>>>>> --- a/xen/common/domctl.c
>>>>>>> +++ b/xen/common/domctl.c
>>>>>>> @@ -172,7 +172,7 @@ void getdomaininfo(struct domain *d, struct
>>>>> xen_domctl_getdomaininfo *info)
>>>>>>>             info->max_vcpu_id = v->vcpu_id;
>>>>>>>             if ( !test_bit(_VPF_down, &v->pause_flags) )
>>>>>>>             {
>>>>>>> -            if ( !(v->pause_flags & VPF_blocked) )
>>>>>>> +            if ( !test_bit(_VPF_blocked, &v->pause_flags) )
>>>>>>
>>>>>> test_bit() is quite a bit more complex an operation than a simple &,
>>>>>> and with (on x86) even constant_test_bit() involving a cast to
>>>>>> a pointer to volatile I'm afraid we can't even hope that compilers
>>>>>> would produce identical code for both in cases like this one (as that
>>>>>> casts limits freedom of the compiler). IOW I'd rather see other
>>>>>> test_bit(_VPF_...) uses converted the inverse way (which as a nice
>>>>>> but minor side effect would yield slightly smaller source code).
>>>>>
>>>>> What about introducing __test_bit() being a variant which can be
>>>>> reordered by omitting the volatile modifier? I think this would have
>>>>> the same effect.
>>>>
>>>> I'm not convinced it always would - the inline function is still more
>>>> complex than the plain operation.
>>>
>>> Depends on the way it is done. What about:
>>>
>>> #define __test_bit(nr, addr) ({                         \
>>>       if ( bitop_bad_size(addr) ) __bitop_bad_size();     \
>>>       (__builtin_constant_p(nr) ?                         \
>>>        !!(*(addr) & ((typeof)(*(addr))1 << (nr))) :       \
>>>        __variable_test_bit((nr),(addr)));                 \
>>> })
>>
>> But that's not correct - addr may point to wider than a single entry
>> array, irrespective of whether nr is a compile time constant.
>>
>>> It would even be possible to drop the test for bitop_bad_size(addr) in
>>> the constant case.
>>
>> In which case 1 << nr may reference a bit beyond the type
>> of *addr.
> 
> Hmm, yes, you are right, of course.
> 
> It could be fixed, however.
> 
> The question is: does it make sense to follow this path any longer,
> or would you reject it even in case of correctness? I wouldn't mind
> either way, I just don't want to waste time (mine and yours).

I continue to think that the better route would be to get rid of the
unnecessary test_bit() uses in favor of the shorter and less
restrictive (to the compiler) & operation.

Jan


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