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

[Ingo Molnar <mingo@kernel.org>]

* Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> Mon, Apr 21, 2025 at 08:51:47PM +0200, Ingo Molnar kirjoitti:
> > 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.
> 
> ...
> 
> > +/*
> > + * 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);
> 
> I would add some spaces here and there for the sake of readability.

I think it's perfectly readable, skipping the whitespace for numeric 
literals is standard style. Linus himself does that occasionally, see:

  94a2bc0f611c ("arm64: add 'runtime constant' support")

  static inline void __runtime_fixup_ptr(void *where, unsigned long val)
  {
           __le32 *p = lm_alias(where);
           __runtime_fixup_16(p, val);
           __runtime_fixup_16(p+1, val >> 16);
           __runtime_fixup_16(p+2, val >> 32);
           __runtime_fixup_16(p+3, val >> 48);
           __runtime_fixup_caches(where, 4);
  }

Or:

  938df695e98d ("vsprintf: associate the format state with the format pointer")

  +       unsigned int shift = 32 - size*8;

which uses visual grouping to make arithmethic expressions more 
readable.

> 
> > +		return;
> > +	}
> > +	if (size < SZ_1G) {
> 
> Can be written in one line as
> 
> 	} else if (...) {

Done. (See delta patch below.)

> 
> Ditto for the rest.
> 
> > +		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);
> > +}
> 
> Don't you want to use string_helpers.h provided API? 
> string_get_size().

I don't think string_get_size() knows the fine distinction between:

    BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff]  256   KB device reserved

and:

    BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff]  256.0 KB device reserved

"256 KB" is exactly 256 KB, while "256.0 KB" denotes a value that is a 
bit larger than 256 KB but rounds down to 256 KB at 1 KB granularity.

When reading platform boot logs it's useful to know when such values 
are exact, at a glance.

Thanks,

	Ingo

====================>
 arch/x86/kernel/e820.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 7f600d32a999..67a477203c97 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -213,22 +213,19 @@ static void e820_print_size(u64 size)
 		else
 			pr_cont(" %4llu   KB", size/SZ_1K);
 		return;
-	}
-	if (size < SZ_1G) {
+	} else 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) {
+	} else 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))
+	} else 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);