linux-kernel - Re: [PATCH linux-next] arm64: kexec: Support the case of VA_BITS=39 in trans_pgd_idmap

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Message-ID: <YeroVxXEcUz8RTgv@arm.com>
Date:   Fri, 21 Jan 2022 17:07:35 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     cgel.zte@...il.com
Cc:     pasha.tatashin@...een.com, si.hao@....com.cn,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, james.morse@....com
Subject: Re: [PATCH linux-next] arm64: kexec: Support the case of VA_BITS=39
 in trans_pgd_idmap_page()

On Fri, Jan 21, 2022 at 06:52:16AM +0000, cgel.zte@...il.com wrote:
> From: sihao <si.hao@....com.cn>
> 
> When the values of CONFIG_ARM64_VA_BITS and CONFIG_ARM64_PA_BITS are not
> equal, the following panic occurs when kexec is executed.
> 
> This happens because trans_pgd_idmap_page() does not support VA_BITS=39.
> So the patch supports the case of VA_BITS=39.
[...]
> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index d7da8ca40d2e..3d88185adcf5 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -232,7 +232,7 @@ int trans_pgd_idmap_page(struct trans_pgd_info *info, phys_addr_t *trans_ttbr0,
>  {
>  	phys_addr_t dst_addr = virt_to_phys(page);
>  	unsigned long pfn = __phys_to_pfn(dst_addr);
> -	int max_msb = (dst_addr & GENMASK(52, 48)) ? 51 : 47;

This should have been GENMASK(51, 48), though it doesn't make any
difference and may work better with the change below:

> +	int max_msb = (dst_addr & GENMASK(52, VA_BITS)) ? 51 : (VA_BITS - 1);

So when VA_BITS == 52, the mask is 0 and we set max_msb to 51.

I wonder, could we use fls64() instead here?

-- 
Catalin