Received-SPF: pass (google.com: domain of linux-kernel+bounces-60051-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Date: Fri, 9 Feb 2024 13:46:52 -0800
In-Reply-To: <CAHk-=wgBt9SsYjyHWn1ZH5V0Q7P6thqv_urVCTYqyWNUWSJ6_g@mail.gmail.com>
Precedence: bulk
Mime-Version: 1.0
References: <20240208220604.140859-1-seanjc@google.com> <CAKwvOdk_obRUkD6WQHhS9uoFVe3HrgqH5h+FpqsNNgmj4cmvCQ@mail.gmail.com>
 <DM6PR02MB40587AD6ABBF1814E9CCFA7CB84B2@DM6PR02MB4058.namprd02.prod.outlook.com>
 <CAHk-=wi3p5C1n03UYoQhgVDJbh_0ogCpwbgVGnOdGn6RJ6hnKA@mail.gmail.com>
 <ZcZyWrawr1NUCiQZ@google.com> <CAKwvOdmKaYYxf7vjvPf2vbn-Ly+4=JZ_zf+OcjYOkWCkgyU_kA@mail.gmail.com>
 <CAHk-=wgEABCwu7HkJufpWC=K7u_say8k6Tp9eHvAXFa4DNXgzQ@mail.gmail.com> <CAHk-=wgBt9SsYjyHWn1ZH5V0Q7P6thqv_urVCTYqyWNUWSJ6_g@mail.gmail.com>
Message-ID: <ZcadTKwaSvvywNA9@google.com>
Subject: Re: [PATCH] Kconfig: Explicitly disable asm goto w/ outputs on gcc-11
 (and earlier)
From: Sean Christopherson <seanjc@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>, Uros Bizjak <ubizjak@gmail.com>, 
	Jakub Jelinek <jakub@redhat.com>, "Andrew Pinski (QUIC)" <quic_apinski@quicinc.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Masahiro Yamada <masahiroy@kernel.org>, 
	Peter Zijlstra <peterz@infradead.org>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Content-Type: text/plain; charset="us-ascii"

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.