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

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Date:   Wed, 12 Aug 2020 19:45:12 +0530
From:   Gaurav Kohli <gkohli@...eaurora.org>
To:     Will Deacon <will@...nel.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 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@...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
> 

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