From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support
Date: Fri, 20 Oct 2017 17:20:28 +0200
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Cc: Nicolas Pitre <nicolas.pitre@linaro.org>, Michal Hocko <mhocko@suse.com>,
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	Alexei Starovoitov <ast@kernel.org>,
	virtualization@lists.linux-foundation.org,
	David Howells <dhowells@redhat.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Waiman Long <longman@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Christoph Lameter <cl@linux.com>, Thomas Gleixner <tglx@linutronix.de>,
	the arch/x86 maintainers <x86@kernel.org>,
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	Daniel Borkmann <daniel@iogearbox.net>, Jon
To: Thomas Garnier <thgarnie@google.com>
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* Thomas Garnier <thgarnie@google.com> 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,

	Ingo