linux-kernel - Re: [PATCH 0/7] remove size limit on XIP kernel

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Message-ID: <46c70799-87b1-4478-95bb-4d4c90519f5b@ghiti.fr>
Date: Sun, 12 May 2024 17:47:24 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Nam Cao <namcao@...utronix.de>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] remove size limit on XIP kernel

Hi Nam,

On 10/05/2024 08:28, Nam Cao wrote:
> Hi,
>
> For XIP kernel, the writable data section is always at offset specified in
> XIP_OFFSET, which is hard-coded to 32MB.
>
> Unfortunately, this means the read-only section (placed before the
> writable section) is restricted in size. This causes build failure if the
> kernel gets too large.
>
> This series remove the use of XIP_OFFSET one by one, then remove this
> macro entirely at the end, with the goal of lifting this size restriction.
>
> Also some cleanup and documentation along the way.
>
> This series depends on
>      https://lore.kernel.org/linux-riscv/20240508191917.2892064-1-namcao@linutronix.de/
> to apply cleanly, and also depends on
>      https://lore.kernel.org/linux-riscv/20240508173116.2866192-1-namcao@linutronix.de/
> which fixes a boot issue.
>
> Best regards,
> Nam
>
> Nam Cao (7):
>    riscv: cleanup XIP_FIXUP macro
>    riscv: replace va_kernel_pa_offset with va_kernel_data_pa_offset on
>      XIP
>    riscv: drop the use of XIP_OFFSET in XIP_FIXUP_OFFSET
>    riscv: drop the use of XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET
>    riscv: drop the use of XIP_OFFSET in kernel_mapping_va_to_pa()
>    riscv: drop the use of XIP_OFFSET in create_kernel_page_table()
>    riscv: remove limit on the size of read-only section for XIP kernel
>
>   arch/riscv/include/asm/page.h       | 25 ++++++++++++++++++------
>   arch/riscv/include/asm/pgtable.h    | 18 +++++++----------
>   arch/riscv/include/asm/xip_fixup.h  | 30 +++++++++++++++++++++++------
>   arch/riscv/kernel/vmlinux-xip.lds.S |  4 ++--
>   arch/riscv/mm/init.c                | 11 +++++++----
>   5 files changed, 59 insertions(+), 29 deletions(-)
>

XIP kernels are intended for use with flash devices so the XIP_OFFSET 
restriction actually represents the size of the flash device: IIRC this 
32MB was chosen because it would fit "most devices". I think it would be 
good to come up with a mechanism that allows to restrict the size at 
build time: a config? XIP kernels are custom kernels so the user could 
enter its flash size so that if kernel ends up being too large, it 
fails. And by default, we could a very large size to avoid kernel test 
robot build failures.

Thanks,

Alex