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 15:00, Jan Beulich wrote:
> On 05.05.2026 13:46, Orzel, Michal wrote:
>> 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.
> 
> And as also said before, this really is poor man's offset compression then. 
> That
> may be tolerable if you insist that's best for Arm, yet then I'd suggest to 
> limit
> that offset to just the "no compression" case. It's redundant with offset
> compression, and it may be (possible to make) redundant with mask compression.
> If the latter can't be arranged for, an offset may want introducing there as 
> well.
> But it shouldn't exist independent of the compression scheme used.
Having a single per-scheme mechanism rather than an extra independent offset is
cleaner. But I don't think we can limit frametable_base_pdx to PDX_NONE today:

 - Mask compression doesn't fold a leading [0, first_ram_pdx) zero
   prefix into anything. So the PDX of
   the first RAM frame stays at first_ram_pdx, and without the offset
   the frame table virtual extent is max_pdx * sizeof(page_info)
   rather than (max_pdx - first_ram_pdx) * sizeof(page_info).

   For systems with a high RAM base (the 5TB example I gave earlier
   needs ~70GB just to skip the leading hole, vs. 32GB FRAMETABLE_SIZE
   on arm64) the (max_pdx > FRAMETABLE_NR) check then fails and we
   panic before mapping anything. pdx_group_valid (which patch 2/2
   adds) avoids backing those leading groups with physical memory, but
   it doesn't shrink the virtual extent — only the offset does.

 - With offset compression you're right that the leading hole could be
   absorbed into the lookup table, making the extra offset redundant.
   But Arm doesn't currently select offset compression, it's non-selectable,
untested and switching
   the default is a separate (and bigger) discussion that I don't think
   should block this fix given the state of the release.

So as it stands, the offset is needed on Arm for both PDX_NONE and
PDX_MASK_COMPRESSION. Folding it into the mask scheme (and dropping it
for offset compression) is a reasonable cleanup, but it's a refactor
of the compression layer itself, not something I'd like to mix into
this series.


~Michal