Re: [PATCH] x86: work around clang IAS bug referencing __force_order

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Zhenzhong Duan <zhenzhong.duan@oracle.com>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Juergen Gross <jgross@suse.com>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Will Deacon <will@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
Date: Sat, 22 Aug 2020 17:17:05 -0400	[thread overview]
Message-ID: <20200822211705.GA1382515@rani.riverdale.lan> (raw)
In-Reply-To: <CANiq72=3iOh5Z_rboZf-_GUpBE2LUUv3yPKOVNN_XmaNswdNSw@mail.gmail.com>

On Sat, Aug 22, 2020 at 08:17:32PM +0200, Miguel Ojeda wrote:
> On Sat, Aug 22, 2020 at 11:52 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > I am asking myself who is using such ancient compilers?
> 
> There are many users/companies using older versions of compilers,
> kernels and everything. GCC <= 4.9 will still be used/supported (by
> third parties) for a handful of years at least.
> 
> However, the important question is whether those users/companies care
> about running the latest kernels. Many of those definitely do not want
> to touch their kernel either. For those that do, there are several
> longterms to pick from that still support 4.9, as well as other
> workarounds.
> 
> Thus I am usually in favor of raising the minimum whenever new hacks
> are required to be added. On the other hand, we already raised the
> version twice this year and it is not clear to me what is the minimum
> version we would need to go for to ensure this does not bite us.
> 
> > If this is a real problem with GCC version <= 5, so can this be moved
> > to a GCC specific include header-file?
> > Thinking of include/linux/compiler-gcc.h or
> > include/linux/compiler_types.h with a GCC-VERSION check?
> 
> That would be better if it can be done, yes.
> 
> Cheers,
> Miguel

The fix landed in gcc 6.5, 7.3 and 8.1. The bug is presumably quite
difficult to actually trigger. As a sample data point, I verified that
7.1 vs 7.1+fix have no differences on 32-bit and 64-bit x86 defconfigs,
on current mainline.

Assuming we don't want to risk removing force_order, I'd suggest
- make it an input/output operand, so it enforces ordering fully.
- either restrict it to gcc < 8, or just provide a proper definition in
  some file (maybe arch/x86/kernel/cpu/common.c)?

Thanks.