linux-kernel - [patch part-III V2 02/23] x86/entry/64: Avoid pointless code when CONTEXT

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Message-Id: <20200308231718.634477991@linutronix.de>
Date:   Mon, 09 Mar 2020 00:14:12 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [patch part-III V2 02/23] x86/entry/64: Avoid pointless code when CONTEXT_TRACKING=n

From: Thomas Gleixner <tglx@...utronix.de>

GAS cannot optimize out the test and conditional jump when context tracking
is disabled and CALL_enter_from_user_mode is an empty macro.

Wrap it in #ifdeffery. Will go away once all this is moved to C.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@...cle.com>
Acked-by: Frederic Weisbecker <frederic@...nel.org>
Acked-by: Andy Lutomirski <luto@...nel.org>

---
 arch/x86/entry/entry_64.S |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -884,12 +884,14 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work
 	TRACE_IRQS_OFF
 	.endif
 
+#ifdef CONFIG_CONTEXT_TRACKING
 	.if \paranoid == 0
 	testb	$3, CS(%rsp)
 	jz	.Lfrom_kernel_no_context_tracking_\@
 	CALL_enter_from_user_mode
 .Lfrom_kernel_no_context_tracking_\@:
 	.endif
+#endif
 
 	movq	%rsp, %rdi			/* pt_regs pointer */