linux-kernel - Re: [PATCH] riscv: Start of DRAM should at least be aligned on PMD size for the direct mapping

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Date:   Thu, 06 Jul 2023 10:05:54 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     alexghiti@...osinc.com
CC:     Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
        suagrfillet@...il.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, alexghiti@...osinc.com
Subject:     Re: [PATCH] riscv: Start of DRAM should at least be aligned on PMD size for the direct mapping

On Tue, 04 Jul 2023 05:18:37 PDT (-0700), alexghiti@...osinc.com wrote:
> So that we do not end up mapping the whole linear mapping using 4K
> pages, which is slow at boot time, and also very likely at runtime.
>
> So make sure we align the start of DRAM on a PMD boundary.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> ---
>  arch/riscv/mm/init.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 4fa420faa780..4a43ec275c6d 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -214,8 +214,13 @@ static void __init setup_bootmem(void)
>  	memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
>
>  	phys_ram_end = memblock_end_of_DRAM();
> +
> +	/*
> +	 * Make sure we align the start of the memory on a PMD boundary so that
> +	 * at worst, we map the linear mapping with PMD mappings.
> +	 */
>  	if (!IS_ENABLED(CONFIG_XIP_KERNEL))
> -		phys_ram_base = memblock_start_of_DRAM();
> +		phys_ram_base = memblock_start_of_DRAM() & PMD_MASK;

This rounds down, which IIUC will result in mappings outside what 
memblock detected as the start af DRAM.  I'd expect that to cause bad 
behavior somewhere.

Shouldn't we be rounding up?

>
>  	/*
>  	 * In 64-bit, any use of __va/__pa before this point is wrong as we