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Re: [PATCH v2 4/8] pdx: introduce command line compression toggle
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 25 Jun 2025 18:00:48 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 25 Jun 2025 16:01:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.06.2025 17:46, Roger Pau Monné wrote:
> On Tue, Jun 24, 2025 at 03:40:16PM +0200, Jan Beulich wrote:
>> On 20.06.2025 13:11, Roger Pau Monne wrote:
>>> Introduce a command line option to allow disabling PDX compression. The
>>> disabling is done by turning pfn_pdx_add_region() into a no-op, so when
>>> attempting to initialize the selected compression algorithm the array of
>>> ranges to compress is empty.
>>
>> While neat, this also feels fragile. It's not obvious that for any
>> algorithm pfn_pdx_compression_setup() would leave compression disabled
>> when there are zero ranges. In principle, if it was written differently
>> for mask compression, there being no ranges could result in compression
>> simply squeezing out all of the address bits. Yet as long as we think
>> we're going to keep this in mind ...
>
> It seemed to me that nr_rages == 0 (so no ranges reported) should
> result in no compression, for example on x86 this means there's no
> SRAT.
Just to mention it: While in the pfn_pdx_compression_setup() flavor in
patch 3 there's no explicit check (hence the logic is assumed to be
coping with that situation), the one introduced in the last patch does
have such an explicit check. Apparently there the logic doesn't cleanly
cover that case all by itself.
Jan
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