Re: [PATCH 2/2 v2] kvm: powerpc: set cache coherency only for kernel managed pages

[Scott Wood <scottwood@freescale.com>]

On 07/18/2013 12:32:18 PM, Alexander Graf wrote:
> 
> On 18.07.2013, at 19:17, Scott Wood wrote:
> 
> > On 07/18/2013 08:19:03 AM, Bharat Bhushan wrote:
> > Likewise, we want to make sure this matches the host entry.   
> Unfortunately, this is a bit of a mess already.  64-bit booke appears  
> to always set MAS2_M for TLB0 mappings.  The initial KERNELBASE  
> mapping on boot uses M_IF_SMP, and the settlbcam() that (IIRC)  
> replaces it uses _PAGE_COHERENT.  32-bit always uses _PAGE_COHERENT,  
> except that initial KERNELBASE mapping.  _PAGE_COHERENT appears to be  
> set based on CONFIG_SMP || CONFIG_PPC_STD_MMU (the latter config  
> clears _PAGE_COHERENT in the non-CPU_FTR_NEED_COHERENT case).
> >
> > As for what we actually want to happen, there are cases when we  
> want M to be set for non-SMP.  One such case is AMP, where CPUs may  
> be sharing memory even if the Linux instance only runs on one CPU  
> (this is not hypothetical, BTW).  It's also possible that we  
> encounter a hardware bug that requires MAS2_M, similar to what some  
> of our non-booke chips require.
> 
> How about we always set M then for RAM?

M is like I in that bad things happen if you mix them.  So we really  
want to match exactly what the rest of the kernel is doing.

Plus, the performance penalty on some single-core chips can be pretty  
bad.

-Scott