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Date: Sat, 22 Aug 2020 17:17:05 -0400 From: Arvind Sankar <nivedita@...m.mit.edu> To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com> Cc: Sedat Dilek <sedat.dilek@...il.com>, Segher Boessenkool <segher@...nel.crashing.org>, Arvind Sankar <nivedita@...m.mit.edu>, Thomas Gleixner <tglx@...utronix.de>, Nick Desaulniers <ndesaulniers@...gle.com>, "Paul E. McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Zhenzhong Duan <zhenzhong.duan@...cle.com>, Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>, Juergen Gross <jgross@...e.com>, Andy Lutomirski <luto@...nel.org>, Andrew Cooper <andrew.cooper3@...rix.com>, LKML <linux-kernel@...r.kernel.org>, clang-built-linux <clang-built-linux@...glegroups.com>, Will Deacon <will@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order 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@...il.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.
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