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

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Date:	Wed, 06 May 2015 13:41:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Denys Vlasenko <dvlasenk@...hat.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 12:09 PM, Denys Vlasenko wrote:
>>
>> 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
> 

That almost certainly is due to paravirt_fail, because otherwise cpuid
would be inline, and gcc actually knows how to optimize around the cpuid
instruction to the point of eliminating the temporaries.

That being said, it would have been better to use a structure.

	-hpa

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