linux-kernel - Re: [PATCH] Kconfig: Explicitly disable asm goto w/ outputs on gcc-11 (and earlier)

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Date: Fri, 9 Feb 2024 13:46:52 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>, Uros Bizjak <ubizjak@...il.com>, 
	Jakub Jelinek <jakub@...hat.com>, "Andrew Pinski (QUIC)" <quic_apinski@...cinc.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] Kconfig: Explicitly disable asm goto w/ outputs on gcc-11
 (and earlier)

On Fri, Feb 09, 2024, Linus Torvalds wrote:
> Sean? Does this work for the case you noticed?

Yep.  You can quite literally see the effect of the asm("").  A "good" sequence
directly propagates the result from the VMREAD's destination register to its
final destination

  <+1756>:  mov    $0x280e,%r13d
  <+1762>:  vmread %r13,%r13
  <+1766>:  jbe    0x209fa <sync_vmcs02_to_vmcs12+1834>
  <+1768>:  mov    %r13,0xe8(%rbx)

whereas the "bad" sequence bounces through a different register.

  <+1780>:  mov    $0x2810,%eax
  <+1785>:  vmread %rax,%rax
  <+1788>:  jbe    0x209e4 <sync_vmcs02_to_vmcs12+1812>
  <+1790>:  mov    %rax,%r12
  <+1793>:  mov    %r12,0xf0(%rbx)

> That (b) is very much voodoo programming, but it matches the old magic
> barrier thing that Jakub Jelinek suggested for the really *old* gcc
> bug wrt plain (non-output) "asm goto". The underlying bug for _that_
> was fixed long ago:
> 
>     http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
> 
> We removed that for plain "asm goto" workaround a couple of years ago,
> so "asm_volatile_goto()" has been a no-op since June 2022, but this
> now resurrects that hack for the output case.
> 
> I'm not loving it, but Sean seemed to confirm that it fixes the code
> generation problem, so ...

Yeah, I'm in the same boat.