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

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Message-ID: <CD2DC486-F4B1-4043-82BC-0CB2AA513A99@nvidia.com>
Date: Tue, 15 Oct 2024 13:42:22 -0400
From: Zi Yan <ziy@...dia.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden <greg.marsden@...cle.com>,
 Ivan Ivanov <ivan.ivanov@...e.com>, Kalesh Singh <kaleshsingh@...gle.com>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Matthias Brugger <mbrugger@...e.com>, Miroslav Benes <mbenes@...e.cz>,
 Oliver Upton <oliver.upton@...ux.dev>, Will Deacon <will@...nel.org>,
 kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 57/57] arm64: Enable boot-time page size selection

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@....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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