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Subject: Re: [PATCH 02/10] powerpc/xive: guest exploitation of the XIVE
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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: =?ISO-8859-1?Q?C=E9dric?= Le Goater <clg@kaod.org>, David Gibson
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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.

Cheers,
Ben.