linux-kernel - Re: [PATCH v3 1/2] arm64: refactor the rodata=xxx

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Message-ID: <83d01335-c6be-d17b-511a-378ce3ad0663@gentwo.org>
Date: Mon, 9 Dec 2024 09:05:04 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Huang Shijie <shijie@...amperecomputing.com>
cc: catalin.marinas@....com, will@...nel.org, anshuman.khandual@....com, 
    corbet@....net, patches@...erecomputing.com, akpm@...ux-foundation.org, 
    thuth@...hat.com, rostedt@...dmis.org, xiongwei.song@...driver.com, 
    ardb@...nel.org, inux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
    linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/2] arm64: refactor the rodata=xxx

On Mon, 9 Dec 2024, Huang Shijie wrote:

> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5901,7 +5901,7 @@
>  	rodata=		[KNL,EARLY]
>  		on	Mark read-only kernel memory as read-only (default).
>  		off	Leave read-only kernel memory writable for debugging.
> -		full	Mark read-only kernel memory and aliases as read-only
> +		noalias	Use more block mappings,may have better performance.
>  		        [arm64]

Maybe use
		noalias Do not check aliases in order to allow larger kernel page
                        sizes on a platforms without FEAT_BBM2 support[arm64])

instead?

>
>  	rockchip.usb_uart
> diff --git a/arch/arm64/include/asm/setup.h b/arch/arm64/include/asm/setup.h
> index ba269a7a3201..b3063326b79a 100644
> --- a/arch/arm64/include/asm/setup.h
> +++ b/arch/arm64/include/asm/setup.h
> @@ -13,6 +13,27 @@
>  extern phys_addr_t __fdt_pointer __initdata;
>  extern u64 __cacheline_aligned boot_args[4];
>
> +/*
> + * rodata=on (default):
> + *    Apply read-only attributes of VM areas to the linear alias of
> + *    the backing pages as well. This prevents code or read-only data
> + *    from being modified (inadvertently or intentionally) via another
> + *    mapping of the same memory page.
> + *
> + *    This requires the linear region to be mapped down to pages,
> + *    which may adversely affect performance in some cases.
> + *
> + * rodata=off:
> + *    It provides us more block mappings and contiguous hits
> + *    to map the linear region which minimize the TLB footprint.
> + *    Leave read-only kernel memory writable for debugging.
> + *
> + * rodata=noalias:
> + *    It provides us more block mappings and contiguous hits
> + *    to map the linear region which minimize the TLB footprint.
> + *    And the linear aliases of pages belonging to read-only mappings
> + *    in vmalloc region are also marked as read-only.
> + */
>  static inline bool arch_parse_debug_rodata(char *arg)
>  {
>  	extern bool rodata_enabled;
> @@ -21,7 +42,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  	if (!arg)
>  		return false;
>
> -	if (!strcmp(arg, "full")) {
> +	if (!strcmp(arg, "on")) {
>  		rodata_enabled = rodata_full = true;
>  		return true;
>  	}
> @@ -31,7 +52,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  		return true;
>  	}
>
> -	if (!strcmp(arg, "on")) {
> +	if (!strcmp(arg, "noalias")) {
>  		rodata_enabled = true;
>  		rodata_full = false;
>  		return true;
>