linux-kernel - Re: [PATCH 2/6] x86: bug: prevent gcc distortions

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Message-ID: <5C4B73C8-7BFC-4473-9ABD-6E23DD79A09A@vmware.com>
Date:   Fri, 18 May 2018 14:30:28 +0000
From:   Nadav Amit <namit@...are.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH 2/6] x86: bug: prevent gcc distortions

Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Thu, May 17, 2018 at 09:13:58AM -0700, Nadav Amit wrote:
>>> +asm(".macro __BUG_FLAGS ins:req file:req line:req flags:req size:req\n"
>>> +    "1:\t \\ins\n\t"
>>> +    ".pushsection __bug_table,\"aw\"\n"
>>> +    "2:\t "__BUG_REL(1b)		"\t# bug_entry::bug_addr\n\t"
>>> +    __BUG_REL(\\file)			"\t# bug_entry::file\n\t"
>>> +    ".word \\line"			"\t# bug_entry::line\n\t"
>>> +    ".word \\flags"			"\t# bug_entry::flags\n\t"
>>> +    ".org 2b+\\size\n\t"
>>> +    ".popsection\n\t"
>>> +    ".endm");
>>> +
>>> +#define _BUG_FLAGS(ins, flags)                                          \
>>> do {									\
>>> +	asm volatile("__BUG_FLAGS \"" ins "\" %c0 %c1 %c2 %c3"		\
>>> +		     : : "i" (__FILE__), "i" (__LINE__),                \
>>> +			 "i" (flags),                                   \
>>> 			 "i" (sizeof(struct bug_entry)));		\
>>> } while (0)
>> 
>> This is an awesome hack, but is there really nothing we can do to make
>> it more readable? Esp, that global asm doing the macro definition is a
>> pain to read.
>> 
>> Also, can we pretty please used named operands in 'new' code?
> 
> Yes, that's my main worry too about all these inlining changes:
> the very, very marked reduction in the readability of assembly code.
> 
> It's bad to an extent that I'm questioning the wisdom of pandering to a compiler 
> limitation to begin with?
> 
> How about asking GCC for an attribute where we can specify the inlined size of an 
> asm() function? Even if we'll just approximate it due to some vagaries of actual 
> code generation related to how arguments interact with GCC, an explicit byte value 
> will do a heck of a better job of it than some sort of implied, vague 'number of 
> newlines' heuristics ...

If it were to become a GCC feature, I think it is best to be a builtin that
says: consider the enclosed expression as “free”. The problem of poor
inlining decisions is not specific to inline asm. As I mentioned in the RFC,
when there are two code paths for constants and variables based on
__builtin_constant_p(), you can get the “cost” of the constant path for
variables.

It is not hard to add such a feature to GCC, but I don’t know how easy it is
to get new features into the compiler.