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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: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: "Orzel, Michal" <michal.orzel@xxxxxxx>
- Date: Tue, 5 May 2026 08:48:15 +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: Tue, 05 May 2026 06:48:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04-May-26 17:28, Roger Pau Monné wrote:
> 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.
The offset is needed regardless of whether compression is used. With
CONFIG_PDX_NONE (no compression, PDX == MFN), if RAM starts at e.g.
0x80000000, the first valid PDX is 0x80000. Without frametable_base_pdx
the frame table would have to be indexed from 0, wasting
0x80000 * sizeof(page_info) of memory just to cover the hole before RAM.
So frametable_base_pdx is really a frame table indexing offset, not
something tied to the compression algorithm.
~Michal
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