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

[Brian Gerst]

On Thu, Feb 14, 2019 at 2:34 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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;

Yes, this looks good.

--
Brian Gerst