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Date: Fri, 20 Oct 2017 17:20:28 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Kees Cook <keescook@...omium.org>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Matthias Kaehlcke <mka@...omium.org>,
	Tom Lendacky <thomas.lendacky@....com>,
	Andy Lutomirski <luto@...nel.org>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Borislav Petkov <bp@...e.de>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Juergen Gross <jgross@...e.com>, Chris Wright <chrisw@...s-sol.org>,
	Alok Kataria <akataria@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
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Subject: Re: [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support


* Thomas Garnier <thgarnie@...gle.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

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