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

[Juergen Gross <jgross@suse.com>]

On 09/28/2015 08:22 AM, Jan Beulich wrote:
>>>> On 28.09.15 at 07:23, <JGross@suse.com> wrote:
>> On 09/25/2015 05:42 PM, Jan Beulich wrote:
>>>>>> On 25.09.15 at 13:54, <JGross@suse.com> wrote:
>>>> Per-VCPU pause flags in sched.h are defined as bit positions and as
>>>> values derived from the bit defines. There is only one user of a value
>>>> which can be easily converted to use a bit number as well.
>>>
>>> I'm not convinced:
>>>
>>>> --- 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)));                 \
})

It would even be possible to drop the test for bitop_bad_size(addr) in
the constant case.

>> And we could still get rid of many double definitions
>> of the same bit. Even if the mask definition of a bit is not error prone
>> by relying on the definition of the bit position, it makes it harder to
>> find all users of this bit.
>
> Why so? Just omit the leading underscore when grep-ing, and you'll
> find all instances (less preprocessor token concatenation, but that's
> orthogonal).

I do use grep for this purpose occasionally, but I prefer tools like
cscope. BTW: IMO using grep the way you are suggesting here is annoying
for cases where the search string is contained in other items.

Juergen