linux-kernel - Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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Date:   Thu, 14 Feb 2019 20:34:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        "H. Peter Anvin" <hpa@...or.com>, valentin.schneider@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

On Thu, Feb 14, 2019 at 11:18:01AM -0500, Brian Gerst wrote:

> > --- a/arch/x86/include/asm/switch_to.h
> > +++ b/arch/x86/include/asm/switch_to.h
> > @@ -40,6 +40,7 @@ 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;
> 
> flags should be initialized in copy_thread_tls().  I think the new
> stack is zeroed already, but it would be better to explicitly set it.

Ah indeed. I somehow misread that code and thought we got initialized
with a copy of current.

Something like the below, right?

---

--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -127,6 +127,7 @@ int copy_thread_tls(unsigned long clone_
 	struct task_struct *tsk;
 	int err;
 
+	frame->flags = 0;
 	frame->bp = 0;
 	frame->ret_addr = (unsigned long) ret_from_fork;
 	p->thread.sp = (unsigned long) fork_frame;
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -392,6 +392,7 @@ int copy_thread_tls(unsigned long clone_
 	childregs = task_pt_regs(p);
 	fork_frame = container_of(childregs, struct fork_frame, regs);
 	frame = &fork_frame->frame;
+	frame->flags = 0;
 	frame->bp = 0;
 	frame->ret_addr = (unsigned long) ret_from_fork;
 	p->thread.sp = (unsigned long) fork_frame;