Re: [Xen-devel] [PATCH v3 1/1] xen: move TLB-flush filtering out into populate_physmap during vm creation

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Wei Liu writes ("Re: [PATCH v3 1/1] xen: move TLB-flush filtering out into 
populate_physmap during vm creation"):
> On Thu, Sep 08, 2016 at 01:01:40PM +0200, Dario Faggioli wrote:
> > On Thu, 2016-09-08 at 11:50 +0100, Wei Liu wrote:
> > > On Thu, Sep 08, 2016 at 01:30:03PM +0800, Dongli Zhang wrote:
> > > > +    if ( next->domain->already_scheduled == 0 )
> > > > +        next->domain->already_scheduled = 1;
> > > > +
> > > Can be simplified by omitting the "if" altogether.  
> > >
> > Are you sure? I mean looking at the cases when the flag is already true
> > (which means, during the life of a domain, basically **always** except
> > a handful of instances after creation), what costs less, a check that
> > is always false, or a write that is always updating a value with its
> > current value?
> 
> Omitting the check certain results in less instructions. And it would
> probably eliminate misses in instruction cache and branch prediction
> logic in the processor.
> 
> In the grand scheme of things, this is a rather minor optimisation, so I
> wouldn't argue strongly for this.

Are we sure we ought to be discussing this in terms of optimisation ?
I doubt it makes any significant difference either way.

But there is a difference in clarity.  I would not normally expect to
see this:

   bool x;

   ...

   if (!x)
       x = 1;

If I saw that I would wonder if the programmer was confused, or
whether I was missing something.

Looking at it without the benefit of the definition of x, it looks
more like x might be a non-boolean type.

Thanks,
Ian.

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