Re: [PATCH for-4.18 v5] xen/pdx: Make CONFIG_PDX_COMPRESSION a common Kconfig option

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Oct 16, 2023 at 03:19:22PM +0200, Jan Beulich wrote:
> On 06.10.2023 17:27, Roger Pau Monné wrote:
> > On Fri, Oct 06, 2023 at 04:09:19PM +0100, Julien Grall wrote:
> >> On 06/10/2023 15:44, Andrew Cooper wrote:
> >>> From: Alejandro Vallejo <alejandro.vallejo@cloud.com>
> >>>
> >>> Adds a new compile-time flag to allow disabling PDX compression and
> >>> compiles out compression-related code/data. It also shorts the pdx<->pfn
> >>> conversion macros and creates stubs for masking functions.
> >>>
> >>> While at it, removes the old arch-defined CONFIG_HAS_PDX flag.  Despite the
> >>> illusion of choice, it was not optional.
> >>>
> >>> There are ARM and PPC platforms with sparse RAM banks - leave compression
> >>> active by default there.  OTOH, there are no known production x86 systems with
> >>> sparse RAM banks, so disable compression.  This decision can be revisited if
> >>> such a platform comes along.
> >>
> >> (Process remarks rather than the code itself)
> >>
> >> Jan is away this week so I want to make sure this doesn't go in without him
> >> having a say.
> >>
> >> While I don't particularly care about the approach taken for x86, Jan voiced
> >> concerned with this approach and so far I didn't see any conclusion. If
> >> there is any, then please point me to them.
> >>
> >> For the record, the objections from Jan are in [1]. If we want to ignore
> >> them, then I think we need a vote. Possibly only from the x86 folks (?).
> > 
> > I would be fine in leaving the option to be selected if we knew that
> > such x86 systems might be feasible, but so far we have seen 0 x86
> > systems with sparse RAM.  That said, I don't have a strong opinion, but
> > the hiding on x86 seems fine to me.  Interested parties can always
> > forcefully select the option, and a case can be made to make it
> > available again on Kconfig.
> 
> I find it odd to demand people to change source code for aspects like
> this. The very least I'd expect is that BIGMEM configurations (which
> I've never seen any production use of) can actually also engage PDX.

So we expect BIGMEM to have sparse RAM regions?  I would have expected
systems with >16TB of RAM to still be contiguous.

Thanks, Roger.