> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Thursday, July 18, 2013 10:48 PM
> To: Bhushan Bharat-R65777
> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; agraf@xxxxxxx; Bhushan Bharat-
> R65777
> Subject: Re: [PATCH 2/2 v2] kvm: powerpc: set cache coherency only for kernel
> managed pages
>
> On 07/18/2013 08:19:03 AM, Bharat Bhushan wrote:
> > If there is a struct page for the requested mapping then it's normal
> > RAM and the mapping is set to "M" bit (coherent, cacheable) otherwise
> > this is treated as I/O and we set "I + G" (cache inhibited, guarded)
> >
> > This helps setting proper TLB mapping for direct assigned device
> >
> > Signed-off-by: Bharat Bhushan <bharat.bhushan@xxxxxxxxxxxxx>
> > ---
> > v2: some cleanup and added comment
> > -
> > arch/powerpc/kvm/e500_mmu_host.c | 23 ++++++++++++++++++-----
> > 1 files changed, 18 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/e500_mmu_host.c
> > b/arch/powerpc/kvm/e500_mmu_host.c
> > index 1c6a9d7..02eb973 100644
> > --- a/arch/powerpc/kvm/e500_mmu_host.c
> > +++ b/arch/powerpc/kvm/e500_mmu_host.c
> > @@ -64,13 +64,26 @@ static inline u32 e500_shadow_mas3_attrib(u32
> > mas3, int usermode)
> > return mas3;
> > }
> >
> > -static inline u32 e500_shadow_mas2_attrib(u32 mas2, int usermode)
> > +static inline u32 e500_shadow_mas2_attrib(u32 mas2, pfn_t pfn)
> > {
> > + u32 mas2_attr;
> > +
> > + mas2_attr = mas2 & MAS2_ATTRIB_MASK;
> > +
> > + /*
> > + * RAM is always mappable on e500 systems, so this is identical
> > + * to kvm_is_mmio_pfn(), just without its overhead.
> > + */
> > + if (!pfn_valid(pfn)) {
>
> Please use page_is_ram(), which is what gets used when setting the WIMG for the
> host userspace mapping. We want to make sure the two are consistent.
>
> > + /* Pages not managed by Kernel are treated as I/O, set
> > I + G */
> > + mas2_attr |= MAS2_I | MAS2_G;
> > #ifdef CONFIG_SMP
> > - return (mas2 & MAS2_ATTRIB_MASK) | MAS2_M;
> > -#else
> > - return mas2 & MAS2_ATTRIB_MASK;
> > + } else {
> > + /* Kernel managed pages are actually RAM so set "M" */
> > + mas2_attr |= MAS2_M;
> > #endif
>
> 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.
Scott, can you please point to the code where MAS2_M is always set for TLB0?
-Bharat
> 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.
>
> -Scott
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