Re: [Xen-devel] [v2 1/6] Port WARN_ON_ONCE() from Linux

[Sameer Goel <sameer.goel@xxxxxxxxxx>]

On 05/25/2018 01:07 AM, Jan Beulich wrote:
> > > > On 24.05.18 at 22:23, <sameer.goel@xxxxxxxxxx> wrote:
> > > > --- a/xen/include/xen/lib.h
> > > > +++ b/xen/include/xen/lib.h
> > > > @@ -11,6 +11,19 @@
> > > >    #define BUG_ON(p)  do { if (unlikely(p)) BUG();  } while (0)
> > > >    #define WARN_ON(p) do { if (unlikely(p)) WARN(); } while (0)
> > > >    
> > > > +#define WARN_ON_ONCE(p)                               \
> > > > +({                                                    \
> > > > +    static bool __section(".data.unlikely") warned;   \
> > > Linux uses .data.once. That or .data.cold would seem better to me than
> > > .data.unlikely.
> > I guess there is not reason to keep this in a specific section. I'll
> > just go ahead and remove the section here?
> There certainly is a reason: We don't want such variables to sit in the
> middle of an otherwise frequently accessed cache line. Hence the "cold"
> part of the suggested alternatives name.
Till the last release Linux was using .data.unlikely to just keep this 
var in its own data section. I can change the name to cold. In Linux 
this was changed to once to enable a sys node that can reset the value 
of this section and for Xen this is not needed.
Thanks,
Sameer
> Jan

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