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

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Date:   Wed, 12 Aug 2020 14:30:44 +0100
From:   Will Deacon <will@...nel.org>
To:     Gaurav Kohli <gkohli@...eaurora.org>
Cc:     linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        neeraju@...eaurora.org
Subject: Re: [PATCH] arm64: Skip apply SSBS call for non SSBS system

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@...eaurora.org>
> 
> 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