Re: [PATCH] arm64: Skip apply SSBS call for non SSBS system

[Gaurav Kohli]

On 8/12/2020 7:00 PM, Will Deacon wrote:
> On Tue, Aug 04, 2020 at 07:44:42PM +0530, Gaurav Kohli wrote:
> > In a system where no cpu's implement SSBS, for
> > them no need to set pstate. This might help to save
> > few cpu cycles during context switch.
> >
> > Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>
> >
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 6089638..79f80f1 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -477,6 +477,13 @@ static void ssbs_thread_switch(struct task_struct *next)
> >   	struct pt_regs *regs = task_pt_regs(next);
> >   
> >   	/*
> > +	 * For Targets which don't have SSBS support, they
> > +	 * can return from here.
> > +	 */
> > +	if (!IS_ENABLED(CONFIG_ARM64_SSBD))
> > +		return;
>
> Does this actually make a measurable difference?
>
> Will

Hi Will,

While doing code review between older kernel and latest kernel for 
context switch case, there i have found this and thought it is good to 
have for non-ssbs system to return early(as this might improve).

Please let me know if you want to run some tests.

Regards
Gaurav