Re: [PATCH v2 1/2] xen/pdx: account for frametable_base_pdx in generic pdx_to_page/page_to_pdx

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Apr 30, 2026 at 02:51:02PM +0200, Michal Orzel wrote:
> The generic pdx_to_page() and page_to_pdx() macros in xen/pdx.h assume
> the frame table starts at PDX 0, which is only true on x86. ARM
> uses a non-zero frametable_base_pdx to offset into the frame table (PPC also
> defines it).
> 
> Fix the generic macros to subtract/add frametable_base_pdx, defaulting
> to 0 when the arch does not define it. This makes the generic macros
> correct for all architectures, even though they are only used on x86
> today.

Hm, I assume this offset was added because the original mask PDX
compression won't (usually) compress the gap between 0 and the start
of RAM.  However the newish offset PDX compression should be able to
compress from 0 to start of RAM, and hence you don't need to apply
an extra PDX offset there?

If that's indeed the case it might be better to integrate
frametable_base_pdx into the mask compression algorithm itself, so
that on some arches it's a mask plus a decrease.

Thanks, Roger.