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

Brian Gerst
2019-02-15T14:34:48Z

On Thu, Feb 14, 2019 at 2:34 PM Peter Zijlstra <peterz@infradead.org> 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