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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.05.2026 12:46, Orzel, Michal wrote:
> On 05-May-26 12:40, Jan Beulich wrote:
>> On 05.05.2026 09:35, Orzel, Michal wrote:
>>> On 05-May-26 09:13, Roger Pau Monné wrote:
>>>> On Tue, May 05, 2026 at 08:48:15AM +0200, Orzel, Michal wrote:
>>>>> 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.
>>>>
>>>> OK, so you are doing some (kind of) address space compression (removing
>>>> the leading empty range to the first RAM region) even when PDX is
>>>> disabled.
>>>>
>>>>> 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.
>>>>
>>>> But you don't really "waste" memory, just address space?  Oh, maybe
>>>> not on ARM as it doesn't use pdx_group_valid?  And so you
>>>> unconditionally populate the frametable from PDX 0 to max PDX.
>>> With pdx_group_valid (which this series adds) we wouldn't waste
>>> physical memory for the leading gap. But we'd still waste virtual address
>>> space and the FRAMETABLE_NR check (max_pdx > FRAMETABLE_NR) becomes tighter
>>> because the full range from PDX 0 must fit. For example with RAM starting 
>>> at 5TB
>>> the virtual offset before the first usable entry would be ~70GB — more than 
>>> the
>>> entire 32GB FRAMETABLE_SIZE on ARM64.
>>
>> Yet still - this is exactly one of the situations offset compression means
>> to cover. I'm entirely with Roger as to it being undesirable to build a
>> special case variant of "offset compression" into "no compression".
> In this case, if you don't want to generalize the macros, how should we 
> proceed
> on Arm if we still need the offset to cover the PDX_NONE variant that we also
> use? In v1 I just created a local override but Julien wanted to generalize the
> macros instead. The discussion about switching the default on Arm from mask to
> offset that is not even selectable on Arm needs to wait for the new release 
> cycle.

I'm not convinced of that. If you need offset by default, why not enable it by
default (right now, and potentially even as a backport if there's any bug that
is being fixed)?

Jan