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

[Scott Wood <scottwood@freescale.com>]

On 07/21/2013 11:39:45 PM, Bhushan Bharat-R65777 wrote:
> 
> 
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Thursday, July 18, 2013 11:09 PM
> > To: Alexander Graf
> > Cc: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org;  
> kvm@vger.kernel.org; Bhushan
> > Bharat-R65777
> > Subject: Re: [PATCH 2/2 v2] kvm: powerpc: set cache coherency only  
> for kernel
> > managed pages
> >
> > 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.
> 
> I am trying to list the invalid mixing of WIMG:
> 
>  1) I & M
>  2) W & I
>  3) W & M (Scott mentioned that he observed issues when  mixing these  
> two)
>  4) is there any other?

That's not what I was talking about (and I don't think I mentioned W at  
all, though it is also potentially problematic).  I'm talking about  
mixing I with not-I (on two different virtual addresses pointing to the  
same physical), M with not-M, etc.

> >  So we really want to
> > match exactly what the rest of the kernel is doing.
> 
> How the rest of kernel is doing is a bit complex. IIUC, if we forget  
> about the boot state then this is how kernel set WIMG bits:
>  1) For Memory always set M if CONFIG_SMP set.
> 	- So KVM can do same. "M" will not be mixed with "W" and "I". G  
> and E are guest control.

I don't think this is accurate for 64-bit.  And what about the AMP case?

-Scott