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Re: [PATCH v2 1/2] xen/pdx: account for frametable_base_pdx in generic pdx_to_page/page_to_pdx
- To: Michal Orzel <michal.orzel@xxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 4 May 2026 17:28:29 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Timothy Pearson <tpearson@xxxxxxxxxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>
- Delivery-date: Mon, 04 May 2026 15:28:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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