Re: [PATCH 07/29] x86/boot/e820: Print out sizes of E820 memory ranges

[Mike Rapoport <rppt@kernel.org>]

On Mon, Apr 21, 2025 at 08:51:47PM +0200, Ingo Molnar wrote:
> Before:
> 
>         BIOS-provided physical RAM map:
>         BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
>         BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
>         BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
>         BIOS-e820: [mem 0x0000000000100000-0x000000007ffdbfff] usable
>         BIOS-e820: [mem 0x000000007ffdc000-0x000000007fffffff] reserved
>         BIOS-e820: [mem 0x00000000b0000000-0x00000000bfffffff] reserved
>         BIOS-e820: [mem 0x00000000fed1c000-0x00000000fed1ffff] reserved
>         BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
>         BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
>         BIOS-e820: [mem 0x000000fd00000000-0x000000ffffffffff] reserved
> 
> After:
> 
> 	BIOS-provided physical RAM map:
> 	BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff]  639   KB kernel usable RAM
> 	BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff]    1   KB reserved
> 	BIOS-e820: [gap 0x00000000000a0000-0x00000000000effff]  320   KB ...
> 	BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff]   64   KB reserved
> 	BIOS-e820: [mem 0x0000000000100000-0x000000007ffdbfff]    1.9 GB kernel usable RAM
> 	BIOS-e820: [mem 0x000000007ffdc000-0x000000007fffffff]  144   KB reserved
> 	BIOS-e820: [gap 0x0000000080000000-0x00000000afffffff]  768   MB ...
> 	BIOS-e820: [mem 0x00000000b0000000-0x00000000bfffffff]  256   MB reserved
> 	BIOS-e820: [gap 0x00000000c0000000-0x00000000fed1bfff] 1005.1 MB ...
> 	BIOS-e820: [mem 0x00000000fed1c000-0x00000000fed1ffff]   16   KB reserved
> 	BIOS-e820: [gap 0x00000000fed20000-0x00000000feffbfff]    2.8 MB ...
> 	BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff]   16   KB reserved
> 	BIOS-e820: [gap 0x00000000ff000000-0x00000000fffbffff]   15.7 MB ...
> 	BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff]  256   KB reserved
> 	BIOS-e820: [gap 0x0000000100000000-0x000000fcffffffff] 1008   GB ...
> 	BIOS-e820: [mem 0x000000fd00000000-0x000000ffffffffff]   12   GB reserved
> 
> Note how a 1-digit precision field is printed out if a range is
> fractional in its largest-enclosing natural size unit.
> 
> So the "256 MB" and "12 GB" fields above denote exactly 256 MB and
> 12 GB regions, while "1.9 GB" signals the region's fractional nature
> and it being just below 2GB.
> 
> Printing E820 maps with such details visualizes 'weird' ranges
> at a glance, and gives users a better understanding of how
> large the various ranges are, without having to perform hexadecimal
> subtraction in their minds.
> 
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Cc: Andy Shevchenko <andy@kernel.org>
> Cc: Arnd Bergmann <arnd@kernel.org>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
> (cherry picked from commit d1ac6b8718575a7ea2f0a1ff347835a8879df673)
> ---
>  arch/x86/kernel/e820.c | 47 +++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index 10bd10bd5672..8ee89962fcbf 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -199,6 +199,41 @@ static void __init e820_print_type(enum e820_type type)
>  	}
>  }
>  
> +/*
> + * Print out the size of a E820 region, in human-readable
> + * fashion, going from KB, MB, GB to TB units.
> + *
> + * Print out fractional sizes with a single digit of precision.
> + */
> +static void e820_print_size(u64 size)
> +{
> +	if (size < SZ_1M) {
> +		if (size & (SZ_1K-1))
> +			pr_cont(" %4llu.%01llu KB", size/SZ_1K, 10*(size & (SZ_1K-1))/SZ_1K);
> +		else
> +			pr_cont(" %4llu   KB", size/SZ_1K);
> +		return;
> +	}

I'd make this a helper, e.g

static void __e820_print_size(u64 size, u64 unit, const char *unit_name)
{
	if (size & (unit - 1)) {
		u64 fraction = 10 * (size & (unit - 1))/unit;

		pr_cont(" %4llu.%01llu %s", size/unit, fraction, unit_name);
	} else {
		pr_cont(" %4llu   %s", size/unit, unit_name);
	}
}

> +	if (size < SZ_1G) {
> +		if (size & (SZ_1M-1))
> +			pr_cont(" %4llu.%01llu MB", size/SZ_1M, 10*(size & (SZ_1M-1))/SZ_1M);
> +		else
> +			pr_cont(" %4llu   MB", size/SZ_1M);
> +		return;
> +	}
> +	if (size < SZ_1T) {
> +		if (size & (SZ_1G-1))
> +			pr_cont(" %4llu.%01llu GB", size/SZ_1G, 10*(size & (SZ_1G-1))/SZ_1G);
> +		else
> +			pr_cont(" %4llu   GB", size/SZ_1G);
> +		return;
> +	}
> +	if (size & (SZ_1T-1))
> +		pr_cont(" %4llu.%01llu TB", size/SZ_1T, 10*(size & (SZ_1T-1))/SZ_1T);
> +	else
> +		pr_cont(" %4llu   TB", size/SZ_1T);
> +}
> +
>  static void __init e820__print_table(const char *who)
>  {
>  	u64 range_end_prev = 0;
> @@ -215,14 +250,22 @@ static void __init e820__print_table(const char *who)
>  		if (range_start < range_end_prev)
>  			pr_info("BUG: out of order E820 entry!\n");
>  
> +		/* Print gaps, if any: */
>  		if (range_start > range_end_prev) {
> -			pr_info("%s: [gap %#018Lx-%#018Lx]\n",
> +			u64 gap_size = range_start - range_end_prev;
> +
> +			pr_info("%s: [gap %#018Lx-%#018Lx]",
>  				who,
>  				range_end_prev,
>  				range_start-1);
> +
> +			e820_print_size(gap_size);
> +			pr_cont(" ...\n");
>  		}
>  
> -		pr_info("%s: [mem %#018Lx-%#018Lx] ", who, range_start, range_end-1);
> +		/* Print allocated ranges: */
> +		pr_info("%s: [mem %#018Lx-%#018Lx]", who, range_start, range_end-1);
> +		e820_print_size(entry->size);
>  		e820_print_type(entry->type);
>  		pr_cont("\n");
>  
> -- 
> 2.45.2
> 

-- 
Sincerely yours,
Mike.