netdev - Re: [PATCH] be2net: fix adapter->big_page

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Message-ID: <CAKwvOdkCfqfpJYYX+iu2nLCUUkeDorDdVP3e7koB9NYsRwgCNw@mail.gmail.com>
Date:   Thu, 18 Jul 2019 14:10:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Qian Cai <cai@....pw>, Bill Wendling <morbo@...gle.com>,
        James Y Knight <jyknight@...gle.com>
Cc:     David Miller <davem@...emloft.net>, sathya.perla@...adcom.com,
        ajit.khaparde@...adcom.com, sriharsha.basavapatna@...adcom.com,
        somnath.kotur@...adcom.com, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, netdev@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation

On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@....pw> wrote:
>
>
>
> > On Jul 12, 2019, at 8:50 PM, David Miller <davem@...emloft.net> wrote:
> >
> > From: Qian Cai <cai@....pw>
> > Date: Fri, 12 Jul 2019 20:27:09 -0400
> >
> >> Actually, GCC would consider it a const with -O2 optimized level because it found that it was never modified and it does not understand it is a module parameter. Considering the following code.
> >>
> >> # cat const.c
> >> #include <stdio.h>
> >>
> >> static int a = 1;
> >>
> >> int main(void)
> >> {
> >>      if (__builtin_constant_p(a))
> >>              printf("a is a const.\n");
> >>
> >>      return 0;
> >> }
> >>
> >> # gcc -O2 const.c -o const
> >
> > That's not a complete test case, and with a proper test case that
> > shows the externalization of the address of &a done by the module
> > parameter macros, gcc should not make this optimization or we should
> > define the module parameter macros in a way that makes this properly
> > clear to the compiler.
> >
> > It makes no sense to hack around this locally in drivers and other
> > modules.
>
> If you see the warning in the original patch,
>
> https://lore.kernel.org/netdev/1562959401-19815-1-git-send-email-cai@lca.pw/
>
> GCC definitely optimize rx_frag_size  to be a constant while I just confirmed clang
> -O2 does not. The problem is that I have no clue about how to let GCC not to
> optimize a module parameter.
>
> Though, I have added a few people who might know more of compilers than myself.

+ Bill and James, who probably knows more than they'd like to about
__builtin_constant_p and more than other LLVM folks at this point.

-- 
Thanks,
~Nick Desaulniers