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

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 05-May-26 12:49, Jan Beulich wrote:
> 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)?
As said before, we also need offset when using just PDX grouping and no 
compression.

~Michal