[PATCH] powerpc/powernv/prd: Validate whether address to be mapped is part of system RAM

[Jeremy Kerr]

Hi Vaidy,

> The current topic is who owns setting up the ATT bits for that piece
> of memory.  It is the kernel today.  Kernel decides to set this up as
> normal memory or I/O memory and sets the bits in page table entry.
> 
>> Or, what if there's a range of address-space that isn't backed by system
>> RAM (say, some MMIO-mapped hardware) that we want to expose to a future
>> HBRT implementation? This change will block that.
>> 
>> The kernel doesn't know what is and is not valid for a HBRT mapping, so
>> it has no reason to override what's specified in the device tree. We've
>> designed this so that the kernel provides the mechanism for mapping
>> pages, and not the policy of which pages can be mapped.
> 
> The features altered are cache inhibit and guarding which affects
> ability to fetch instructions.  If we allow HBRT to reside in an I/O
> memory, the we need to tell kernel that it is ok to allow caching and
> instruction execution in that region and accordingly change the ATT
> bits.

But this isn't only about the HBRT range itself (ie., the memory 
containing the HBRT binary). Everything that HBRT needs to map will come 
through this path. We may not need to fetch instructions from those ranges.

> This patch does not block a working function, but actually makes
> debugging a failed case easier.  The failing scenario without this
> check is such that HBRT cannot fetch from the region of memory and
> loops in minor page faults doing nothing.

Yep, that's not great, but the alternative means applying this kernel 
policy, which we can't guarantee is correct.

That is, unless the page protection bits mean that this won't work 
anyway, but we can probably fix that without a kernel policy, by 
applying the appropriate pgprot_t, perhaps.

> As Vasant mentioned hostboot team will add code to relocate the HBRT
> to the right place.  Addressing your concern, if we end up allowing
> HBRT in non system-RAM area

Not just HBRT, but anything that HBRT maps too.

> we need to add some more flags in device
> tree to instruct the driver to force change the page protection bits
> as page_prot = pgprot_cached(page_prot);

Doesn't phys_mem_access_prot() handle that for us? Or do I have my 
_noncached/_cached logic inverted?

Cheers,


Jeremy