Re: [PATCH 0/7] x86 vdso32 cleanups

[Andy Lutomirski]

On Sun, Aug 30, 2015 at 2:18 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Sat, Aug 29, 2015 at 12:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Sat, Aug 29, 2015 at 8:20 AM, Brian Gerst <brgerst@xxxxxxxxx> 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.

The replacement is dramatically faster, which means I probably
benchmarked it wrong.  I'll try again in a day or two.

--Andy
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