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Re: [PATCH 0/8] Make PDX compression optional
- To: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 18 Jul 2023 11:33:03 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 18 Jul 2023 09:33:43 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 17.07.2023 18:03, 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.
I disagree with this choice of default for x86. To avoid surprising
downstreams, this should at best be a two-step process: Keep the
default as Y right now, and switch to N a couple of releases later.
But that's only the simple / mechanical side. Considering my earlier
effort to reduce / remove the involved overhead dynamically at
runtime (which you may or may not be aware of; see [2]), I view a
compile time choice as less desirable. At the very least I would
expect some justification towards this build time choice being
acceptable / reasonable despite the earlier effort towards greater
flexibility. Only such would be likely to have me merely defer to
other x86 maintainers, rather than outright objecting.
Jan
[2] https://lists.xenproject.org/archives/html/xen-devel/2018-09/msg01616.html
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