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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan