Re: [PATCH 02/10] powerpc/xive: guest exploitation of the XIVE interrupt controller

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Aug 10, 2017 at 09:36:04PM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2017-08-10 at 09:19 +0200, Cédric Le Goater wrote:
> > > > > > +  /* Perform the acknowledge hypervisor to register cycle */
> > > > > > +  ack = be16_to_cpu(__raw_readw(xive_tima + TM_SPC_ACK_OS_REG));
> > > > > 
> > > > > Why do you need the raw_readw() + be16_to_cpu + mb, rather than one of
> > > > > the higher level IO helpers?
> > > > 
> > > > This is one of the many ways to do MMIOs on the TIMA. This memory 
> > > > region defines a set of offsets and sizes for which loads and 
> > > > stores have different effects. 
> > > > 
> > > > See the arch/powerpc/include/asm/xive-regs.h file and 
> > > > arch/powerpc/kvm/book3s_xive_template.c for some more usage.
> > > 
> > > Sure, much like any IO region.  My point is, why do you want this kind
> > > of complex combo, rather than say an in_be16() or readw_be().
> > > 
> > 
> > The code is inherited from the native backend. 
> > 
> > I think this is because we know what we are doing and we skip 
> > the synchronization routines of the higher level IO helpers. 
> > That might not be necessary for sPAPR. Ben ? 
> 
> It's a performance optimisation, we know we don't need the
> sync,twi,isync crap of the higher level accessor here.

Ok.  A comment mentioning this would be good - unless you're very
familiar with the code and hardware it's not going to be obvious if
the nonstandard IO accessors are for legitimate performance reasons,
or just cargo-culting.


-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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