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Re: [PATCH for-4.18 v5] xen/pdx: Make CONFIG_PDX_COMPRESSION a common Kconfig option
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 16 Oct 2023 15:40:19 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>, Julien Grall <jgrall@xxxxxxxxxx>, Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Henry Wang <Henry.Wang@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Julien Grall <julien@xxxxxxx>
- Delivery-date: Mon, 16 Oct 2023 13:40:41 +0000
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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@xxxxxxxxx>
> >>>
> >>> 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.
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