From: hpa@zytor.com
To: Peter Zijlstra <peterz@infradead.org>, Michael Davidson <md@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Matthias Kaehlcke <mka@chromium.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
grundler@chromium.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Greg Hackmann <ghackmann@google.com>,
Kees Cook <keescook@chromium.org>,
"linux-tip-commits@vger.kernel.org"
<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility
Date: Sun, 07 May 2017 08:42:52 -0700 [thread overview]
Message-ID: <B5C90399-63CA-4F70-8D09-CFB5E55A0D42@zytor.com> (raw)
In-Reply-To: <20170506081635.GE4626@worktop.programming.kicks-ass.net>
On May 6, 2017 1:16:35 AM PDT, Peter Zijlstra <peterz@infradead.org> wrote:
>On Fri, May 05, 2017 at 01:36:34PM -0700, Michael Davidson wrote:
>
>> There are a few lingering places in the kernel which use variable
>> length arrays in structs (eg the raid10 driver) which don't build
>with
>> clang and that is about it.
>
>So the other point I raised is lack of asm goto (and asm flags output).
>
>Without that our static key infrastructure reverts to runtime branches
>and affects performance.
>
>> So, while I completely understand the resistance to adding arbitrary
>> hacks to the kernel just to support another compiler it is important
>> to also understand just how close things are to "just working".
>
>Reading up on the LLVM thread on asm goto they appear to want to
>provide
>an intrinsic to allow doing the patchable branch thing. That would be
>fairly limiting, and the proposal I've seen doesn't even cover the two
>(or rather 4) states of patchable branches we have in the kernel.
>
>Not to mention that such an intrinsic doesn't even begin to cover all
>the other (perhaps creative) uses we have got asm goto used.
>
>But my main point is that we'd have to rewrite and maintain _two_
>versions of the static key infrastructure if we were to support LLVM's
>intrinsic and the GCC asm goto. That is a very undesirable place to be.
>
>So while they'll say they support the feature, I'll say its worthless
>since I'm not inclined to support their variant of it. As is, I'm not
>getting the feeling the LLVM team really cares about Linux.
This has been a common problem with LLVM: they say that they will provide feature compatibility with gcc, but then they say this or that gcc extension "doesn't make sense" and is something they don't want to support.
--
Sent from my Android device with K-9 Mail. Please excuse my brevity.
next prev parent reply other threads:[~2017-05-07 22:00 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-01 22:47 [PATCH v2] x86/mm/kaslr: Use _ASM_MUL macro for multiplication Matthias Kaehlcke
2017-05-02 2:08 ` Kees Cook
2017-05-05 8:11 ` [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility tip-bot for Matthias Kaehlcke
2017-05-05 10:25 ` Peter Zijlstra
2017-05-05 17:50 ` Ingo Molnar
2017-05-05 18:22 ` Peter Zijlstra
2017-05-05 18:44 ` Matthias Kaehlcke
2017-05-05 19:30 ` Linus Torvalds
2017-05-05 20:36 ` Michael Davidson
2017-05-06 8:16 ` Peter Zijlstra
2017-05-07 15:42 ` hpa [this message]
2017-05-05 20:52 ` Matthias Kaehlcke
2017-05-06 9:57 ` Ingo Molnar
2017-05-05 19:37 ` hpa
2017-05-05 21:24 ` Matthias Kaehlcke
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