linux-kernel - Re: [PATCH v2 1/2] Makefile: Fix distcc compilation with x86 macros

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Message-ID: <1F17E69E-06E8-4A5A-A434-1DCD6C6F0259@vmware.com>
Date:   Mon, 19 Nov 2018 17:18:06 +0000
From:   Nadav Amit <namit@...are.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
CC:     Ingo Molnar <mingo@...hat.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] Makefile: Fix distcc compilation with x86 macros

at 8:20 PM, Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> On Sat, Nov 17, 2018 at 6:02 AM Nadav Amit <namit@...are.com> wrote:
>> From: Masahiro Yamada
>> Sent: November 16, 2018 at 7:45:45 AM GMT
>>> To: Nadav Amit <namit@...are.com>
>>> Cc: Ingo Molnar <mingo@...hat.com>, Michal Marek <michal.lkml@...kovi.net>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, H. Peter Anvin <hpa@...or.com>, X86 ML <x86@...nel.org>, Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
>>> Subject: Re: [PATCH v2 1/2] Makefile: Fix distcc compilation with x86 macros
>>> 
>>> 
>>> On Thu, Nov 15, 2018 at 1:01 PM Nadav Amit <namit@...are.com> wrote:
>>>> Introducing the use of asm macros in c-code broke distcc, since it only
>>>> sends the preprocessed source file. The solution is to break the
>>>> compilation into two separate phases of compilation and assembly, and
>>>> between the two concatenate the assembly macros and the compiled (yet
>>>> not assembled) source file. Since this is less efficient, this
>>>> compilation mode is only used when distcc or icecc are used.
>>>> 
>>>> Note that the assembly stage should also be distributed, if distcc is
>>>> configured using "CFLAGS=-DENABLE_REMOTE_ASSEMBLE".
>>>> 
>>>> Reported-by: Logan Gunthorpe <logang@...tatee.com>
>>>> Signed-off-by: Nadav Amit <namit@...are.com>
>>> 
>>> 
>>> Wow, this is so ugly.
>> 
>> Indeed, it is not pretty from the Makefile system point of view.
>> 
>> But it does have merits when it comes to the actual use (by developers). If
>> you look on x86, there are a lot of duplicated implementation for C and Asm
>> and crazy C macros, for example for PV function call or the alternative
>> mechanism. The code is also less readable in its current form.
>> 
>>> I realized how much I hated this by now.
>>> 
>>> My question is, how long do we need to carry this?
>> 
>> If it is purely about performance, I am not sure it would make sense to
>> put it in, as it will indeed be (eventually) solved by the compiler, and
>> the penalty is not too great.
> 
> 
> It is unfortunate to mess up the code
> by having two different ways to achieve the same goal.

Indeed, but I fail to see how this statement fits with the next sentence.

> When GCC with asm inline support is shipped,
> would you revert 77b0bf55 ?

This patch and the following ones were not written to be reverted, i.e.,
reverting them later may not be easy since they remove redundant C inline
assembly chunks.

Since gcc will solve the inline assembly issues, the value of these patches
is in unifying the assembly that is used in .c and .S files. Due to the lack
of m4-like preprocessor in the Linux make-system, the solution is a bit
“hacky” (in other words, I don’t see a reasonable alternative solution).

So there is a downside in the form of performance degradation of distcc, and
hacky (not too hacky?) Makefile changes. On the upside, except addressing
the gcc statement cost computation (inline) issue, the patch removes
redundant code, and improves assembly code readability. In addition, it
provides the option to make assembly manipulations after compilation, which
HPA and others (me) look into.

.Anyhow, if after all, the downside is considered greater than the upside,
it is best to remove the patches sooner than later, as a revert later will
be more painful.

Regards,
Nadav