Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

[hpa]

On May 6, 2017 1:16:35 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
-- 
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