linux-kernel - Re: [PATCH 1/3] ARM: Premit ioremap() to map reserved pages

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Message-ID: <20140122113855.GH1621@mudshark.cambridge.arm.com>
Date:	Wed, 22 Jan 2014 11:38:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Geng Hui <hui.geng@...wei.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] ARM: Premit ioremap() to map reserved pages

On Wed, Jan 22, 2014 at 11:25:14AM +0000, Wang Nan wrote:
> This patch relaxes the restriction set by commit 309caa9cc, which
> prohibit ioremap() on all kernel managed pages.
> 
> Other architectures, such as x86 and (some specific platforms of) powerpc,
> allow such mapping.
> 
> ioremap() pages is an efficient way to avoid arm's mysterious cache control.
> This feature will be used for arm kexec support to ensure copied data goes into
> RAM even without cache flushing, because we found that flush_cache_xxx can't
> reliably flush code to memory.
> 
> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> Cc: <stable@...r.kernel.org> # 3.4+
> Cc: Eric Biederman <ebiederm@...ssion.com>
> Cc: Russell King <rmk+kernel@....linux.org.uk>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Geng Hui <hui.geng@...wei.com>
> ---
>  arch/arm/mm/ioremap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/ioremap.c b/arch/arm/mm/ioremap.c
> index f123d6e..98b1c10 100644
> --- a/arch/arm/mm/ioremap.c
> +++ b/arch/arm/mm/ioremap.c
> @@ -298,7 +298,7 @@ void __iomem * __arm_ioremap_pfn_caller(unsigned long pfn,
>  	/*
>  	 * Don't allow RAM to be mapped - this causes problems with ARMv6+
>  	 */
> -	if (WARN_ON(pfn_valid(pfn)))
> +	if (WARN_ON(pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn))))

Since reserved pages can still be mapped, how does this avoid the cacheable
alias issue fixed by 309caa9cc6ff ("ARM: Prohibit ioremap() on kernel
managed RAM")?

Will
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