Re: [PATCH v3 0/4] Make PDX compression optional

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 16/08/2023 10:43, Jan Beulich wrote:
> On 16.08.2023 11:36, Alejandro Vallejo wrote:
> > On Tue, Aug 08, 2023 at 02:02:16PM +0100, Alejandro Vallejo wrote:
> > > Currently there's a CONFIG_HAS_PDX Kconfig option, but it's impossible to
> > > disable it because the whole codebase performs unconditional
> > > compression/decompression operations on addresses. This has the
> > > unfortunate side effect that systems without a need for compression still
> > > have to pay the performance impact of juggling bits on every pfn<->pdx
> > > conversion (this requires reading several global variables). This series
> > > attempts to:
> > >
> > >    * Leave the state of pdx and pdx compression documented
> > >    * Factor out compression so it _can_ be removed through Kconfig
> > >    * Make it so compression is disabled on x86 and enabled on both Aarch32
> > >      and Aarch64 by default.
> > >
> > > Series summary:
> > >
> > > Patch 1 Moves hard-coded compression-related logic to helper functions
> > > Patch 2 Refactors all instances of regions being validated for pdx
> > >          compression conformance so it's done through a helper
> > > Patch 3 Non-functional reorder in order to simplify the patch 8 diff
> > > Patch 4 Adds new Kconfig option to compile out PDX compression and removes
> > >          the old CONFIG_HAS_PDX, as it was non removable
> > >
> > > Already committed:
> > >
> > > v1/patch 1 documents the current general understanding of the pdx concept and
> > >             pdx compression in particular
> > > v1/patch 3 Marks the pdx compression globals as ro_after_init
> > > v2/patch 1 Documents the differences between arm32 and arm64 directmaps
> > >
> > > Alejandro Vallejo (4):
> > >    mm: Factor out the pdx compression logic in ma/va converters
> > >    mm/pdx: Standardize region validation wrt pdx compression
> > >    pdx: Reorder pdx.[ch]
> > >    pdx: Add CONFIG_PDX_COMPRESSION as a common Kconfig option
> > @Jan: Just making sure, are you generally ok with this series as-is?
> Well, okay would be too strong; I still don't see why my runtime patching
> series isn't re-considered.
Do you have a pointer to the series? I would be interested to have a look.

That said... the problem with alt-patching is this is architectural 
specific. Right now, this seems to be a bit unnecessary given that we 
believe that virtually no-one will have a platform (I know we talked 
about a potential one...) where PDX is compressing.
Cheers,

--
Julien Grall