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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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?

Thanks, Roger.