> -----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; k...@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?
So it mean it is safe to let guest control G and E.
> 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.
2) For I/O , drivers can pass flags to set M or "I + G".
- For KVM; if not memory then it is I/O. For now we can always set "I +
G".
- Later we can design some mechanism in VFIO interface to let KVM
somehow know whether to set "M" or "I+G".
-Bharat
>
> Plus, the performance penalty on some single-core chips can be pretty bad.
>
> -Scott
--
To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
