linux-kernel - Re: [PATCH v6 3/3] x86: paravirt: make native_save

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Date:   Fri, 22 Jun 2018 10:10:16 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     mingo@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, hpa@...or.com,
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        Manoj Gupta <manojgupta@...gle.com>,
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        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
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Subject: Re: [PATCH v6 3/3] x86: paravirt: make native_save_fl extern inline

On Thu, Jun 21, 2018 at 7:24 PM Ingo Molnar <mingo@...nel.org> wrote:
> * Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> > native_save_fl() is marked static inline, but by using it as
> > a function pointer in arch/x86/kernel/paravirt.c, it MUST be outlined.
>
> > --- a/arch/x86/include/asm/irqflags.h
> > +++ b/arch/x86/include/asm/irqflags.h
> > @@ -13,7 +13,7 @@
> >   * Interrupt control:
> >   */
> >
> > -static inline unsigned long native_save_fl(void)
> > +extern inline unsigned long native_save_fl(void)
> >  {
> >       unsigned long flags;
> >
>
> What's the code generation effect of this on say a defconfig kernel vmlinux with
> paravirt enabled?

Starting with this patch set applied:
$ make CC=gcc-8 -j46
$ objdump -d vmlinux | grep native_save_fl --context=3
ffffffff81059140 <native_save_fl>:
ffffffff81059140: 9c                    pushfq
ffffffff81059141: 58                    pop    %rax
ffffffff81059142: c3                    retq
$ git checkout HEAD~3
$ make CC=gcc-8 -j46
$ objdump -d vmlinux | grep native_save_fl --context=3
ffffffff81079410 <native_save_fl>:
ffffffff81079410: 9c                    pushfq
ffffffff81079411: 58                    pop    %rax
ffffffff81079412: c3                    retq

Mainly, this is to prevent the compiler from adding a stack protector
to the outlined version, as the stack protector clobbers %rcx, but
paravirt expects %rcx to be preserved. More info can be found:
https://lkml.org/lkml/2018/5/24/1242--
Thanks,
~Nick Desaulniers