Re: [Xen-devel] [PATCH v3] xen/arm: flush D-cache and I-cache when appropriate

[Tim Deegan <tim@xxxxxxx>]

At 13:56 +0000 on 19 Nov (1353333381), Ian Campbell wrote:
> On Mon, 2012-11-19 at 13:08 +0000, Tim Deegan wrote:
> > Sorry for the late review - I meant to get to these on Thursday but
> > hadn't time.  
> > 
> > At 12:55 +0000 on 19 Nov (1353329757), Ian Campbell wrote:
> > > > +/* Macro for flushing a single small item.  The predicate is always 
> > > > + * compile-time constant so this will compile down to 3 instructions in
> > > > + * the common case. 
> > 
> > This is no longer true!  The predicate now depends on the alignment of
> > the pointer, so this will compile to quite a bit of code, including the
> > memory barrier I was trying to avoid.
> 
> If p is static then isn't p + sizeof(*p) etc static and therefore the
> compiler can figure it out?

No.  Cacheline_bytes isn't known until run time, and even if p is
static, its value isn't known until link time. :(

> Stefano, what does it actually compile down to in practice?
> 
> > If we can sensibly restrict this to naturally-aligned objects smaller
> > than a cacheline, we should go back to the previous version of this
> > macro.  If not, I think we might as well just use
> > flush_xen_dcache_va_range(_p, sizeof *_p) and be done with it.
> 
> I've already committed it, so please send a followup patch of whichever
> sort is needed.

Will do.  Again, probably Thursday before I can get to it.

Tim.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel