[PATCH] powerpc/mm/book3s64/hash: Update 4k PAGE_SIZE kernel mapping

[Benjamin Herrenschmidt]

On Wed, 2019-10-16 at 20:30 +1100, Michael Ellerman wrote:
> I think the main reason is that in some configurations we can't use 64K
> pages for MMIO, so the 64K vmalloc mappings and 4K MMIO mappings need to
> be in different segments.
> 
> It's possible that's no longer true on modern configs. But annoyingly I
> can't easily tell because the kernel doesn't know, it defers to firmware
> telling it via the "ibm,pa-features" property and setting
> MMU_FTR_CI_LARGE_PAGE.
> 
> The spec (PAPR) doesn't give us any hints either it just says the bit in
> the device tree means:
> 
>   The value of 1 indicates support for I=1 (cache inhibited) large
>   pages; else not supported.
> 
> Which at least suggests that firmware is allowed to support 64K MMIO
> mappings or not at its whim.
> 
> Hopefully Ben or Paul remember more details than me.

Not 100% certain but I think the HW only started supporting non-4k MMIO
pages with P7 (or was it some variant of P6 ?)

Also it's unclear under what circumstances the proprietary hypervisor
or KVM will allow it. I know at some point KVM didn't...

Cheers,
Ben.