Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)

From: Andy Lutomirski <luto@amacapital.net>
To: James Y Knight <jyknight@google.com>
Cc: "Yatsina, Marina" <marina.yatsina@intel.com>,
	Kees Cook <keescook@google.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Chandler Carruth <chandlerc@google.com>,
	"Kreitzer, David L" <david.l.kreitzer@intel.com>,
	"Grischenko, Andrei L" <andrei.l.grischenko@intel.com>,
	"rnk@google.com" <rnk@google.com>,
	LLVM Developers <llvm-dev@lists.llvm.org>,
	"ehsan@mozilla.com" <ehsan@mozilla.com>,
	"Tayree, Coby" <coby.tayree@intel.com>,
	Matthias Braun <matze@braunis.de>,
	Dean Michael Berris <dean.berris@gmail.com>,
	Guenter Roeck <linux@roeck-us.net>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
	Rik van Riel <riel@redhat.com>, Andi Kleen <ak@linux.intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jiri Kosina <jikos@kernel.org>,
	"Hansen, Dave" <dave.hansen@intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linux-foundation.org>,
	Paul Turner <pjt@google.com>, Stephen Hines <srhines@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)
Date: Wed, 14 Feb 2018 17:21:55 +0000	[thread overview]
Message-ID: <CALCETrVeOKXa_xiggzqjcx34TtTs_rMyq56u-EXCFYgQtcei5g@mail.gmail.com> (raw)
In-Reply-To: <CAA2zVHqxQeZRdQtfhL-QRjqE5W94g+NU9E9Lx8RwwjDof36M_Q@mail.gmail.com>

On Wed, Feb 14, 2018 at 3:31 PM, James Y Knight <jyknight@google.com> wrote:
> I'd be definitely in favor having clang support asm goto. I wouldn't
> want to exclude having other conversations about how to more directly
> provide compiler features that the linux kernel could use, ALSO, but I
> do not think that conversation should block implementing asm-goto.
>
> IMO, inline asm is, generally, a valuable feature to provide in the
> compiler as an escape hatch, and asm goto is a relatively sane
> extension of it. Supporting outgoing edges from an inline asm block is
> a reasonable thing for users to desire, and as far as anyone's said so
> far, seems like it ought to be fairly easily implementable, without
> causing bad side-effects in the compiler.
>
> Of course, we generally do want to minimize the need for users to use
> inline asm, by providing appropriate compiler support for the features
> people would otherwise be forced to implement using asm. But I don't
> see that as really any more important for asm-goto than any other
> inline asm. There will always be a desire for escape hatches, to do
> weird and unique things which aren't supported directly in the
> compiler. (Also, the kernel is a pretty special case in terms of its
> requirements, it seems exceedingly unlikely that we could ever provide
> enough compiler support to let it eliminate inline asm.).
>

If clang implements asm goto, could you also pretty please make it
better than GCC and support output constraints in the same asm
statements?  This would give us a very clean, short, and fast
implementation for __get_user() and __put_user(), and it would also
help put pressure on GCC to implement the same feature.