linux-kernel - Re: [PATCH 4/4] x86/efi: print size and base in binary units in efi_print

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Date:	Fri, 8 Jan 2016 18:44:55 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	"Elliott, Robert (Persistent Memory)" <elliott@....com>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] x86/efi: print size and base in binary units in efi_print_memmap

On Fri, Jan 8, 2016 at 6:38 PM, Elliott, Robert (Persistent Memory)
<elliott@....com> wrote:
>> -----Original Message-----
>> From: Matt Fleming [mailto:matt@...eblueprint.co.uk]
>> Sent: Friday, January 8, 2016 6:19 AM
>> To: Andy Shevchenko <andy.shevchenko@...il.com>
>> Cc: Elliott, Robert (Persistent Memory) <elliott@....com>; Thomas Gleixner
>> <tglx@...utronix.de>; Ingo Molnar <mingo@...hat.com>; H. Peter Anvin
>> <hpa@...or.com>; x86@...nel.org; linux-efi@...r.kernel.org; linux-
>> kernel@...r.kernel.org
>> Subject: Re: [PATCH 4/4] x86/efi: print size and base in binary units in
>> efi_print_memmap
>>
>> On Sun, 27 Dec, at 04:35:12PM, Andy Shevchenko wrote:
>> > On Mon, Dec 21, 2015 at 6:16 PM, Matt Fleming <matt@...eblueprint.co.uk>
>> wrote:
>> > >> diff --git a/arch/x86/platform/efi/efi.c
>> b/arch/x86/platform/efi/efi.c
>> > >> index 635a955..030ba91 100644
>> > >> --- a/arch/x86/platform/efi/efi.c
>> > >> +++ b/arch/x86/platform/efi/efi.c
>> > >> @@ -222,6 +222,25 @@ int __init efi_memblock_x86_reserve_range(void)
>> > >>       return 0;
>> > >>  }
>> > >>
>> > >> +char * __init efi_size_format(char *buf, size_t size, u64 bytes)
>> > >> +{
>> > >> +     if (!bytes || (bytes & 0x3ff))
>> > >> +             snprintf(buf, size, "%llu B", bytes);
>> > >> +     else if (bytes & 0xfffff)
>> > >> +             snprintf(buf, size, "%llu KiB", bytes >> 10);
>> > >> +     else if (bytes & 0x3fffffff)
>> > >> +             snprintf(buf, size, "%llu MiB", bytes >> 20);
>> > >> +     else if (bytes & 0xffffffffff)
>> > >> +             snprintf(buf, size, "%llu GiB", bytes >> 30);
>> > >> +     else if (bytes & 0x3ffffffffffff)
>> > >> +             snprintf(buf, size, "%llu TiB", bytes >> 40);
>> > >> +     else if (bytes & 0xfffffffffffffff)
>> > >> +             snprintf(buf, size, "%llu PiB", bytes >> 50);
>> > >> +     else
>> > >> +             snprintf(buf, size, "%llu EiB", bytes >> 60);
>> > >> +     return buf;
>> >
>> > For me it looks like ffs with name in the table can be used.
>>
>> Could you provide a patch?
>
> I think this is functionally equivalent:
> #include <string.h>
>
> char * efi_size_format_ffsl(char *buf, size_t size, u64 bytes)
> {
>         if (!bytes || ffsl(bytes) < 10)
>                 snprintf(buf, size, "%llu B", bytes);
>         else if (ffsl(bytes) < 20)
>                 snprintf(buf, size, "%llu KiB", bytes >> 10);
>         else if (ffsl(bytes) < 30)
>                 snprintf(buf, size, "%llu MiB", bytes >> 20);
>         else if (ffsl(bytes) < 40)
>                 snprintf(buf, size, "%llu GiB", bytes >> 30);
>         else if (ffsl(bytes) < 50)
>                 snprintf(buf, size, "%llu TiB", bytes >> 40);
>         else if (ffsl(bytes) < 60)
>                 snprintf(buf, size, "%llu PiB", bytes >> 50);
>         else
>                 snprintf(buf, size, "%llu EiB", bytes >> 60);
>         return buf;
> }

No, no, I meant something slightly different.

We already have a table of units. Needs to be shared (patch already
cooked), second stage is to provide proper number and units.

Something like

units = string_units_2;
units_index = __ffs64(value) / 10;
value >>= units_index * 10;

snprintf("%llu%s", value, units[units_index]);


>
> Compiled as a user program with gcc -O2, the original results
> in mov and testq instructions:
>         movq    %rdi, %rbx
>         je      .L2
>         testl   $1023, %edx
>         jne     .L2
>         testl   $1048575, %edx
>         jne     .L15
>         testl   $1073741823, %edx
>         jne     .L16
>         movabsq $1099511627775, %rax
>         testq   %rax, %rdx
>         jne     .L17
>         movabsq $1125899906842623, %rax
>         testq   %rax, %rdx
>         jne     .L18
>         movabsq $1152921504606846975, %rax
>         movq    %rdx, %rcx
>         testq   %rax, %rdx
>         jne     .L19
>
> while the ffs version uses bit scan forward (bsfq)
> and only needs cmpl instructions since the values
> are smaller:
>         movq    %rdi, %rbx
>         je      .L21
>         bsfq    %rdx, %rcx
>         addq    $1, %rcx
>         cmpl    $9, %ecx
>         jle     .L21
>         cmpl    $19, %ecx
>         jle     .L33
>         cmpl    $29, %ecx
>         jle     .L34
>         cmpl    $39, %ecx
>         .p2align 4,,2
>         jle     .L35
>         cmpl    $49, %ecx
>         .p2align 4,,2
>         jle     .L36
>         cmpl    $59, %ecx
>         .p2align 4,,2
>         jle     .L37
>
> The kernel offers ffs(int x) but not ffsl(),

See above.

> and it
> uses inline assembly for one of these:
>         bsfl
>         bsfl, cmovzl
>         bsfl, jnz, movl
>
> I don't know which code is the most efficient.

-- 
With Best Regards,
Andy Shevchenko