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

[Juergen Gross <jgross@suse.com>]

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. 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.


Juergen