Introduce "ondemand" linker sections #77827
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added 2 commits
August 30, 2024 12:29
This provides __ondemand_func and __ondemand_rodata tags for annotating functions and read-only data that should be loaded on demand so not to exhaust all available RAM. The demand paging mechanism will be leveraged to load so designated code/data as needed and to evict it from memory when unused. Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
This sets initial unpaged mappings for __ondemand_func code and __ondemand_rodata variables. To achieve this, we have to augment the backing store API. Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
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area: Kconfig
area: Linker Scripts
area: ARM64
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dcpleung
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Aug 30, 2024
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Pinned sections can still be used, right?
Well, you have to choose. Either you have explicit pinned sections and
everything else is implicitly evictable, or you have explicit on-demand
(aka evictable) sections and everything else is implicitly pinned. Both
tags may coexist of course.
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Wonder if they should be make mutually exclusive by adding |
dcpleung
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Sep 3, 2024
cfriedt
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Sep 3, 2024
andyross
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Sep 5, 2024
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FWIW I do kind of worry that we've now implemented two converse solutions to the same problem... Clearly the API can be different with how the defaults work, but surely the backend should be the same for "pinned" and "on demand", no?
Not a -1, but sort of a smell complaint.
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Nothing prevents both strategies from using the same backend. But their
initial state must differs.
In the explicit pinned model, everything is initially in memory and
stuff gets evicted when memory is tight. The backing store is used as a
swap space and is initially unpopu;ated. This assumes the firmware may
all fit in memory at boot time.
In the explicit on-demand model, most everything is initially in the
evicted state. The backing store already contains the evicted stuff,
most likely in read-only mode. This is especially useful when the
firmware is bigger than available RAM and no XIP is possible.
Where both strategies differ the most is in their initial MMU mappings.
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@andyross @peter-mitsis ping |
andyross
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Sep 10, 2024
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This is needed when the firmware is significantly bigger than the available
RAM.
This can be seen in action with the ARM64 demand paging support in PR #74129.