Re: [RFC PATCH v1 57/57] arm64: Enable boot-time page size selection

Zi Yan
2024-10-15T17:44:50Z

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On 14 Oct 2024, at 6:59, Ryan Roberts wrote:

> Introduce a new Kconfig, ARM64_BOOT_TIME_PAGE_SIZE, which can be
> selected instead of a page size. When selected, the resulting kernel's
> page size can be configured at boot via the command line.
>
> For now, boot-time page size kernels are limited to 48-bit VA, since
> more work is required to support LPA2. Additionally MMAP_RND_BITS and
> SECTION_SIZE_BITS are configured for the worst case (64K pages). Future
> work could be implemented to be able to configure these at boot time for
> optimial page size-specific values.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---

<snip>

>
> @@ -1588,9 +1601,10 @@ config XEN
> # 4K | 27 | 12 | 15 | 10 |
> # 16K | 27 | 14 | 13 | 11 |
> # 64K | 29 | 16 | 13 | 13 |
> +# BOOT| 29 | 16 (max) | 13 | 13 |
> config ARCH_FORCE_MAX_ORDER
> int
> - default "13" if ARM64_64K_PAGES
> + default "13" if ARM64_64K_PAGES || ARM64_BOOT_TIME_PAGE_SIZE
> default "11" if ARM64_16K_PAGES
> default "10"
> help

So boot-time page size kernel always has the highest MAX_PAGE_ORDER, which
means the section size increases for 4KB and 16KB page sizes. Any downside
for this?

Is there any plan (not in this patchset) to support boot-time MAX_PAGE_ORDER
to keep section size the same?

Best Regards,
Yan, Zi

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