linux-kernel - [PATCH 26/25] sched/x86_64: Don't save flags on context switch

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Message-ID: <20190319111740.GZ6058@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Mar 2019 12:17:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com
Cc:     linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        rostedt@...dmis.org
Subject: [PATCH 26/25] sched/x86_64: Don't save flags on context switch

Subject: sched/x86_64: Don't save flags on context switch
From: Peter Zijlstra <peterz@...radead.org>
Date: Tue Mar 19 11:35:46 CET 2019

Now that we have objtool validating AC=1 state for all x86_64 code,
we can once again guarantee clean flags on schedule.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/entry/entry_64.S        |    2 --
 arch/x86/include/asm/switch_to.h |    2 +-
 arch/x86/kernel/process_64.c     |    7 -------
 3 files changed, 1 insertion(+), 10 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -291,7 +291,6 @@ ENTRY(__switch_to_asm)
 	pushq	%r13
 	pushq	%r14
 	pushq	%r15
-	pushfq
 
 	/* switch stack */
 	movq	%rsp, TASK_threadsp(%rdi)
@@ -314,7 +313,6 @@ ENTRY(__switch_to_asm)
 #endif
 
 	/* restore callee-saved registers */
-	popfq
 	popq	%r15
 	popq	%r14
 	popq	%r13
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -40,13 +40,13 @@ asmlinkage void ret_from_fork(void);
  * order of the fields must match the code in __switch_to_asm().
  */
 struct inactive_task_frame {
-	unsigned long flags;
 #ifdef CONFIG_X86_64
 	unsigned long r15;
 	unsigned long r14;
 	unsigned long r13;
 	unsigned long r12;
 #else
+	unsigned long flags;
 	unsigned long si;
 	unsigned long di;
 #endif
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -393,13 +393,6 @@ int copy_thread_tls(unsigned long clone_
 	fork_frame = container_of(childregs, struct fork_frame, regs);
 	frame = &fork_frame->frame;
 
-	/*
-	 * For a new task use the RESET flags value since there is no before.
-	 * All the status flags are zero; DF and all the system flags must also
-	 * be 0, specifically IF must be 0 because we context switch to the new
-	 * task with interrupts disabled.
-	 */
-	frame->flags = X86_EFLAGS_FIXED;
 	frame->bp = 0;
 	frame->ret_addr = (unsigned long) ret_from_fork;
 	p->thread.sp = (unsigned long) fork_frame;