From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
Andy Lutomirski <luto@kernel.org>,
Alexander Potapenko <glider@google.com>,
Dmitriy Vyukov <dvyukov@google.com>,
Matthias Kaehlcke <mka@chromium.org>,
Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent
Date: Fri, 15 Sep 2017 12:20:52 -0500 [thread overview]
Message-ID: <20170915172052.5kmtzjk44dywk2k6@treble> (raw)
In-Reply-To: <96dcfb0d-75b9-9707-67df-9bae33186b7d@virtuozzo.com>
On Fri, Sep 15, 2017 at 07:53:46PM +0300, Andrey Ryabinin wrote:
>
>
> On 08/31/2017 08:25 PM, Josh Poimboeuf wrote:
> >
> > There have been a few other ideas which have *almost* worked:
> >
> > 1) Make the 'register void *__sp asm(_ASM_SP)' a global variable instead
> > of a local one. This works for GCC and doesn't break clang. However
> > it resulted in a lot of changed code on the GCC side. It looked like
> > some optimizations had been disabled, even in functions which
> > shouldn't have been affected.
> >
>
> (For those who wasn't following previous discussion there is the patch - http://lkml.kernel.org/r/<75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com>
> But, Josh discovered that the patch causes regression in .text size with gcc > 7.x
> http://lkml.kernel.org/r/<20170721132452.ihpws67e3e7ym3al@treble> )
>
> I'm not so sure that this is disabled optimization. I assume that global rsp makes
> changes something in gcc's register allocation logic, or something like that leading
> to subtle changes in generated code.
>
> But what I recently find out, is that this "regression" sometimes is actually improvement in .text size.
> It all depends on .config, e.g:
>
>
> allnoconfig:
> text data bss dec hex filename
> patched 938094 207700 1215752 2361546 2408ca allnoconfig_p/vmlinux
> unpatched 938768 211796 1215752 2366316 241b6c allnoconfig/vmlinux
>
>
> defconfig:
> text data bss dec hex filename
> patched 10691469 4882856 876544 16450869 fb0535 defconfig_p/vmlinux
> unpatched 10691035 4882856 876544 16450435 fb0383 defconfig/vmlinux
>
> allyesconfig:
> text data bss dec hex filename
> patched 159216239 153294411 49692672 362203322 1596c8ba allyesconfig_p/vmlinux
> unpatched 159224951 153294387 49692672 362212010 1596eaaa allyesconfig/vmlinux
>
>
> So there is regression with Josh's config and defconfig, but allnoconfig and allyesconfig shows improvement.
> Note that sometimes there is the difference in .data size too, don't ask me why.
>
> Given that, perhaps the best thing to do here would be asking gcc devs why is generated code changed.
> And if such changes are harmless (as I suspect) we can just fix the problem with this simple patch http://lkml.kernel.org/r/<75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com> ?
> Despite the increased .text on some configs.
Can you change all the other code sites in a similar way, and then run
the numbers again?
$ git grep __sp |grep register |grep void
arch/x86/include/asm/alternative.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/mshyperv.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/mshyperv.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/paravirt_types.h: register void *__sp asm("esp")
arch/x86/include/asm/paravirt_types.h: register void *__sp asm("rsp")
arch/x86/include/asm/preempt.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/preempt.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/processor.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/rwsem.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/uaccess.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/xen/hypercall.h: register void *__sp asm(_ASM_SP);
arch/x86/kvm/emulate.c: register void *__sp asm(_ASM_SP);
arch/x86/kvm/vmx.c: register void *__sp asm(_ASM_SP);
arch/x86/mm/fault.c: register void *__sp asm("rsp");
--
Josh
next prev parent reply other threads:[~2017-09-15 17:20 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-31 14:11 [RFC PATCH 0/4] x86/asm: Add ASM_CALL() macro for inline asms with call instructions Josh Poimboeuf
2017-08-31 14:11 ` [RFC PATCH 1/4] x86/paravirt: Fix output constraint macro names Josh Poimboeuf
2017-08-31 14:11 ` [RFC PATCH 2/4] x86/asm: Convert some inline asm positional operands to named operands Josh Poimboeuf
2017-08-31 14:11 ` [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent Josh Poimboeuf
2017-08-31 16:11 ` Linus Torvalds
2017-08-31 17:25 ` Josh Poimboeuf
2017-08-31 17:31 ` Josh Poimboeuf
2017-09-02 10:32 ` Ingo Molnar
2017-09-14 14:48 ` Josh Poimboeuf
2017-09-14 17:16 ` Linus Torvalds
2017-09-14 17:26 ` Josh Poimboeuf
2017-09-14 17:33 ` Josh Poimboeuf
2017-09-14 18:28 ` Linus Torvalds
2017-09-14 18:45 ` Josh Poimboeuf
2017-09-15 16:10 ` Josh Poimboeuf
2017-09-15 16:53 ` Andrey Ryabinin
2017-09-15 17:20 ` Josh Poimboeuf [this message]
2017-09-15 18:01 ` Linus Torvalds
2017-09-15 23:29 ` Josh Poimboeuf
2017-09-16 22:22 ` Andrey Ryabinin
2017-09-18 17:40 ` Josh Poimboeuf
2017-09-19 16:02 ` Josh Poimboeuf
2017-08-31 14:11 ` [RFC PATCH 4/4] x86/asm: Use ASM_CALL() macro for inline asm statements with call instructions Josh Poimboeuf
2017-08-31 14:50 ` Peter Zijlstra
2017-08-31 15:21 ` Josh Poimboeuf
2017-08-31 15:36 ` Dmitry Vyukov
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