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Re: [Xen-devel] [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support




> On Oct 20, 2017, at 5:20 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> 
> * Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> 
>>>>       */
>>>> -     cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
>>>> +     leaq    .Lentry_SYSCALL_64_after_fastpath_call(%rip), %r11
>>>> +     cmpq    %r11, (%rsp)
>>>>      jne     1f
> 
>>> This patch seems to add extra overhead to the syscall fast-path even when 
>>> PIE is
>>> disabled, right?
>> 
>> It does add extra instructions when one is not possible, I preferred
>> that over ifdefing but I can change it.
> 
> So my problem is, this pattern repeats in many other places as well, but 
> sprinking 
> various pieces of assembly code with #ifdefs would be very bad as well.
> 
> I have no good idea how to solve this.
> 
> Thanks,

Ugh, brain was off.  This is a bit messy. We could use a macro for this, too, I 
suppose.

> 
>    Ingo

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