Re: [PATCH v8 00/16] Add support for Clang LTO

From: Will Deacon <will@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sami Tolvanen <samitolvanen@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	linux-kbuild <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	PCI <linux-pci@vger.kernel.org>, Jian Cai <jiancai@google.com>,
	Kristof Beyls <Kristof.Beyls@arm.com>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
Date: Fri, 4 Dec 2020 09:35:36 +0000	[thread overview]
Message-ID: <20201204093535.GB461@willie-the-truck> (raw)
In-Reply-To: <CAKwvOdnvq=L=gQMv9MHaStmKMOuD5jvffzMedhp3gytYB6R7TQ@mail.gmail.com>

On Thu, Dec 03, 2020 at 02:32:13PM -0800, Nick Desaulniers wrote:
> On Thu, Dec 3, 2020 at 10:23 AM Will Deacon <will@kernel.org> wrote:
> > On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote:
> > > Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
> > > enabled: the external GNU assembler for stand-alone assembly, and
> > > LLVM's integrated assembler for inline assembly. as-instr tests the
> > > external assembler and makes an admittedly reasonable assumption that
> > > the test is also valid for inline assembly.
> > >
> > > I agree that it would reduce confusion in future if we just always
> > > enabled IAS with LTO. Nick, Nathan, any thoughts about this?
> >
> > That works for me, although I'm happy with anything which means that the
> > assembler checks via as-instr apply to the assembler which will ultimately
> > be used.
> 
> I agree with Will.

[...]

> So I'd recommend to Sami to simply make the Kconfig also depend on
> clang's integrated assembler (not just llvm-nm and llvm-ar).  If
> someone cares about LTO with Clang as the compiler but GAS as the
> assembler, then we can revisit supporting that combination (and the
> changes to KCONFIG), but it shouldn't be something we consider Tier 1
> supported or a combination that need be supported in a minimum viable
> product. And at that point we should make it avoid clang's integrated
> assembler entirely (I suspect LTO won't work at all in that case, so
> maybe even considering it is a waste of time).
> 
> One question I have to Will; if for aarch64 LTO will depend on RCpc,
> but RCpc is an ARMv8.3 extension, what are the implications for LTO on
> pre-ARMv8.3 aarch64 processors?

It doesn't depend on RCpc -- we just emit a more expensive instruction
(an RCsc acquire) if the RCpc one is not supported by both the toolchain
and the CPU. So the implication for those processors is that READ_ONCE()
may be more expensive.

Will