linux-kernel - RE: [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry trampoline

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Date:   Fri, 24 Nov 2017 11:39:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...nel.org>, X86 ML <x86@...nel.org>
CC:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>
Subject: RE: [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry
 trampoline

From: Andy Lutomirski
> Sent: 24 November 2017 04:33
...
> +	/*
> +	 * x86 lacks a near absolute jump, and we can't jump to the real
> +	 * entry text with a relative jump, so we fake it using retq.
> +	 */
> +	pushq	$entry_SYSCALL_64_after_hwframe
> +	retq

Don't some of the cpus have hardware logic that follows call and return?
The above will break that logic and slow things down (unless it is an
expected sequence).

Is there a scratch register that can be used for an indirect jump?

	David