linux-kernel - Re: [PATCH 0/7] x86 vdso32 cleanups

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Date:	Sun, 30 Aug 2015 17:18:31 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/7] x86 vdso32 cleanups

On Sat, Aug 29, 2015 at 12:10 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sat, Aug 29, 2015 at 8:20 AM, Brian Gerst <brgerst@...il.com> wrote:
>> This patch set contains several cleanups to the 32-bit VDSO.  The
>> main change is to only build one VDSO image, and select the syscall
>> entry point at runtime.
>
> Oh no, we have dueling patches!
>
> I have a 2/3 finished series that cleans up the AT_SYSINFO mess
> differently, as I outlined earlier.  I've only done the compat and
> common bits (no 32-bit native support quite yet), and it enters
> successfully on Intel using SYSENTER and on (fake) AMD using SYSCALL.
> The SYSRET bit isn't there yet.
>
> Other than some ifdeffery, the final system_call.S looks like this:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/tree/arch/x86/entry/vdso/vdso32/system_call.S?h=x86/entry_compat
>
> The meat is (sorry for whitespace damage):
>
> .text
> .globl __kernel_vsyscall
> .type __kernel_vsyscall,@function
> ALIGN
> __kernel_vsyscall:
> CFI_STARTPROC
> /*
> * Reshuffle regs so that all of any of the entry instructions
> * will preserve enough state.
> */
> pushl %edx
> CFI_ADJUST_CFA_OFFSET 4
> CFI_REL_OFFSET edx, 0
> pushl %ecx
> CFI_ADJUST_CFA_OFFSET 4
> CFI_REL_OFFSET ecx, 0
> movl %esp, %ecx
>
> #ifdef CONFIG_X86_64
> /* If SYSENTER is available, use it. */
> ALTERNATIVE_2 "", "sysenter", X86_FEATURE_SYSENTER32, \
>                  "syscall",  X86_FEATURE_SYSCALL32
> #endif
>
> /* Enter using int $0x80 */
> movl (%esp), %ecx
> int $0x80
> GLOBAL(int80_landing_pad)
>
> /* Restore ECX and EDX in case they were clobbered. */
> popl %ecx
> CFI_RESTORE ecx
> CFI_ADJUST_CFA_OFFSET -4
> popl %edx
> CFI_RESTORE edx
> CFI_ADJUST_CFA_OFFSET -4
> ret
> CFI_ENDPROC
>
> .size __kernel_vsyscall,.-__kernel_vsyscall
> .previous
>
> And that's it.
>
> What do you think?  This comes with massively cleaned up kernel-side
> asm as well as a test case that actually validates the CFI directives.
>
> Certainly, a bunch of your patches make sense regardless, and I'll
> review them and add them to my queue soon.
>
> --Andy

How does the performance compare to the original?  Looking at the
disassembly, there are two added function calls, and it reloads the
args from the stack instead of just shuffling registers.

--
Brian Gerst
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