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

[Juergen Gross <jgross@xxxxxxxx>]

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:
> > > > > > 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)));                 \
> > })
> 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).

> > > > 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.
> There may be cases, yes, but surely not this one: How likely is it for
> some other variable name to include, say, VPF_blocked?
I think we both agree that [_]VPF_blocked poses no problem using grep.

It's more a matter of taste and what people are used to. If someone
is using cscope for that purpose on a regular basis he will either have
to search for two different variables or he will have to use another
tool, possibly after seeing the definition of VPF_blocked depending on
_VPF_blocked. Both cases will be annoying as an extra action is required
for him.


Juergen

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