Re: [PATCH 2/2] kvm: powerpc: set cache coherency only for kernel managed pages

[Scott Wood <scottwood@freescale.com>]

On 07/24/2013 04:39:59 AM, Alexander Graf wrote:
> 
> On 24.07.2013, at 11:35, Gleb Natapov wrote:
> 
> > On Wed, Jul 24, 2013 at 11:21:11AM +0200, Alexander Graf wrote:
> >>> Are not we going to use page_is_ram() from   
> e500_shadow_mas2_attrib() as Scott commented?
> >>
> >> rWhy aren't we using page_is_ram() in kvm_is_mmio_pfn()?
> >>
> >>
> > Because it is much slower and, IIRC, actually used to build pfn map  
> that allow
> > us to check quickly for valid pfn.
> 
> Then why should we use page_is_ram()? :)
> 
> I really don't want the e500 code to diverge too much from what the  
> rest of the kvm code is doing.

I don't understand "actually used to build pfn map...".  What code is  
this?  I don't see any calls to page_is_ram() in the KVM code, or in  
generic mm code.  Is this a statement about what x86 does?

On PPC page_is_ram() is only called (AFAICT) for determining what  
attributes to set on mmaps.  We want to be sure that KVM always makes  
the same decision.  While pfn_valid() seems like it should be  
equivalent, it's not obvious from the PPC code that it is.

If pfn_valid() is better, why is that not used for mmap?  Why are there  
two different names for the same thing?

-Scott