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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan


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