linux-kernel - Re: [PATCH] modpost: add allow list for llvm IPSCCP

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Date:   Thu, 30 Sep 2021 10:13:42 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] modpost: add allow list for llvm IPSCCP

On 30/09/2021 02.18, Nick Desaulniers wrote:
> On Wed, Sep 29, 2021 at 4:28 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>

> Though for the defconfig case...somehow the cost is more than with the
> sanitizers...
> 
> arch/x86/mm/amdtopology.c:157:7: remark: '__nodes_weight' not inlined
> into 'amd_numa_init' because too costly to inline (cost=930,
> threshold=45) [-Rpass-missed=inline]
>         if (!nodes_weight(numa_nodes_parsed))
>              ^
> 
> Looking at the output of `make LLVM=1 -j72
> arch/x86/mm/amdtopology.ll`, @__nodes_weight is just some inline asm
> (.altinstructions). I wonder if I need to teach the cost model about
> `asm inline`...

Remind me, does clang understand 'asm inline("foo")'? Regardless, it
seems that the

  asm (ALTERNATIVE("call __sw_hweight32", ...
  asm (ALTERNATIVE("call __sw_hweight64", ...

in arch/x86/include/asm/arch_hweight.h could/should be made asm_inline
at least for gcc's sake.

Somewhat related: I really think we should remove __cold from the
definition of __init: It hurts boot time (on a simple board with quite
reproducible boot timing I measured 1-3% some time ago), and it is
likely at least partially responsible for the never-ending tsunami of
functions-that-obviously-should-have-been-inlined(TM) but were not
because the caller is being optimized for size. Whatever small cost in
extra .text is reclaimed after init - and those who are concerned about
the size of the kernel image itself probably build with
CONFIG_OPTIMIZE_FOR_SIZE=y, and I see no change in such an image whether
__init includes __cold or not.

Rasmus