Re: [PATCH v2 2/8] kconfig: turn PDX compression into a choice

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.06.2025 09:49, Roger Pau Monné wrote:
> On Tue, Jun 24, 2025 at 03:13:27PM +0200, Jan Beulich wrote:
>> On 20.06.2025 13:11, Roger Pau Monne wrote:
>>> Rename the current CONFIG_PDX_COMPRESSION to CONFIG_PDX_MASK_COMPRESSION,
>>> and make it part of the PDX compression choice block, in preparation for
>>> adding further PDX compression algorithms.
>>>
>>> No functional change intended as the PDX compression defaults should still
>>> be the same for all architectures, however the choice block cannot be
>>> protected under EXPERT and still have a default choice being
>>> unconditionally selected.  As a result, the new "PDX (Page inDeX)
>>> compression" item will be unconditionally visible in Kconfig.
>>
>> Just to mention it: Afaict there is a functional change, but one I actually
>> appreciate, at least in part. So far ...
>>
>>> --- a/xen/common/Kconfig
>>> +++ b/xen/common/Kconfig
>>> @@ -52,9 +52,10 @@ config EVTCHN_FIFO
>>>  
>>>       If unsure, say Y.
>>>  
>>> -config PDX_COMPRESSION
>>> -   bool "PDX (Page inDeX) compression" if EXPERT && !X86 && !RISCV
>>> -   default ARM || PPC
>>
>> ... for x86 (and RISC-V) this option couldn't be selected. Whereas ...
>>
>>> @@ -67,6 +68,17 @@ config PDX_COMPRESSION
>>>       If your platform does not have sparse RAM banks, do not enable PDX
>>>       compression.
>>>  
>>> +config PDX_MASK_COMPRESSION
>>> +   bool "Mask compression"
>>> +   help
>>> +     Compression relying on all RAM addresses sharing a zeroed bit region.
>>
>> ... this option is now available, as the prior !X86 && !RISCV doesn't
>> re-appear here. (As the description mentions it, that dependency clearly
>> can't appear on the enclosing choice itself.) Since x86 actually still
>> should have mask compression implemented properly, that's fine (from my
>> pov; iirc I even asked that it would have remained available when the
>> earlier change was done), whereas I think for RISC-V it's not quite right
>> to offer the option. It also did escape me why the option was made
>> available for PPC, which I'm pretty sure also lacks the logic to determine
>> a suitable mask.
> 
> Yes, the only architectures that have functional PDX compression are
> x86 and ARM, as neither RISC-V nor PowerPC call the initialization
> functions.  AFAICT this is harmless apart from giving the wrong
> impression to the user that PDX compression might be implemented.
> 
> Would you prefer for me to introduce a new HAS_PDX config option
> that's selected by x86 and ARM, and is used to enable the choice PDX
> config?

Hmm, no, I don't think I want you to make any change to the code. I'm
actually happy with the slight relaxation for x86 (and RISC-V), and
aiui you don't alter behavior for PPC. The fact that behavior there
(and for RISC-V) doesn't look quite right isn't an effect of your
change.

A change may be wanted to the description, to avoid giving the wrong
(afaict) impression of this being "no functional change". Considering
how things ended up the way they are prior to this series, this
becoming explicit may cause _others_ to want you to make changes,
though. Hence I simply wanted to raise that aspect, to give others a
hint that they may need to chime in.

For the record, below is what I think would represent original
behavior ("help" parts omitted), albeit still leaving out the EXPERT
aspect (as it's not clear to me what a condition on a prompt means in
a choice element):

choice
        prompt "PDX (Page inDeX) compression"
        default PDX_MASK_COMPRESSION if !X86 && !RISCV
        default PDX_NONE

config PDX_MASK_COMPRESSION
        bool "Mask compression"
        depends on !X86 && !RISCV

config PDX_NONE
        bool "None"
endchoice

But again, specifically for x86 I'd prefer if PDX_MASK_COMPRESSION
became available (again), so the above is not a suggestion to change
the code, unless others insisted on restoring prior behavior.

Jan