linux-kernel - Re: [PATCH] x86: Deinline cpuid

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Date:	Wed, 06 May 2015 21:09:58 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Deinline cpuid_eax and friends

On 05/06/2015 08:59 PM, H. Peter Anvin wrote:
> On 05/06/2015 10:07 AM, Denys Vlasenko wrote:
>> cpuid_e{a,b,c,d}x() functions compile to 44 bytes of machine code each.
>> On x86 allyesconfig build they have 48 callsites.
>> Deinlining all four of them shrinks kernel by about 1k:
>>
>>    text      data      bss       dec     hex filename
>> 82434909 22255384 20627456 125317749 7783275 vmlinux.before
>> 82433898 22255384 20627456 125316738 7782e82 vmlinux
>>
>> Speed impact: CPUID instruction takes from 50 to 350+ cycles,
>> call overhead is negligible in comparison.
> 
> How on Earth does it make 44 bytes?  Is this due to paravirt_fail?

No, just this construct

        unsigned int eax, ebx, ecx, edx;
        cpuid(op, &eax, &ebx, &ecx, &edx);

is not really that cheap to set up. You need to allocate
variables on stack and take address of each:

ffffffff81063668 <cpuid_eax>:
ffffffff81063668:       55                      push   %rbp
ffffffff81063669:       48 89 e5                mov    %rsp,%rbp
ffffffff8106366c:       48 83 ec 10             sub    $0x10,%rsp
ffffffff81063670:       48 8d 4d fc             lea    -0x4(%rbp),%rcx
ffffffff81063674:       89 7d f0                mov    %edi,-0x10(%rbp)
ffffffff81063677:       48 8d 55 f8             lea    -0x8(%rbp),%rdx
ffffffff8106367b:       48 8d 75 f4             lea    -0xc(%rbp),%rsi
ffffffff8106367f:       48 8d 7d f0             lea    -0x10(%rbp),%rdi
ffffffff81063683:       c7 45 f8 00 00 00 00    movl   $0x0,-0x8(%rbp)
ffffffff8106368a:       e8 3c ff ff ff          callq  ffffffff810635cb <__cpuid>
ffffffff8106368f:       8b 45 f0                mov    -0x10(%rbp),%eax
ffffffff81063692:       c9                      leaveq
ffffffff81063693:       c3                      retq

-- 
vda
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