linux-kernel - Re: [PATCH] arm64: Set SSBS for user threads while creation

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Message-ID: <0c5cd234-5cfb-d093-06e4-a0edb5c68bf8@codeaurora.org>
Date:   Wed, 29 Jan 2020 17:18:53 +0530
From:   Srinivas Ramana <sramana@...eaurora.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     will@...nel.org, maz@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] arm64: Set SSBS for user threads while creation

On 1/2/2020 11:31 PM, Catalin Marinas wrote:
> On Mon, Dec 23, 2019 at 06:32:26PM +0530, Srinivas Ramana wrote:
>> Current SSBS implementation takes care of setting the
>> SSBS bit in start_thread() for user threads. While this works
>> for tasks launched with fork/clone followed by execve, for cases
>> where userspace would just call fork (eg, Java applications) this
>> leaves the SSBS bit unset. This results in performance
>> regression for such tasks.
>>
>> It is understood that commit cbdf8a189a66 ("arm64: Force SSBS
>> on context switch") masks this issue, but that was done for a
>> different reason where heterogeneous CPUs(both SSBS supported
>> and unsupported) are present. It is appropriate to take care
>> of the SSBS bit for all threads while creation itself.
>>
>> Fixes: 8f04e8e6e29c ("arm64: ssbd: Add support for PSTATE.SSBS rather than trapping to EL3")
>> Signed-off-by: Srinivas Ramana <sramana@...eaurora.org>
> 
> I suppose the parent process cleared SSBS explicitly. Isn't the child

Actually we observe that parent(in case of android, zygote that launches 
the app) does have SSBS bit set. However child doesn't have the bit set.

> after fork() supposed to be nearly identical to the parent? If we did as
> you suggest, someone else might complain that SSBS has been set in the
> child after fork().

I am also wondering why would a userspace process clear SSBS bit loosing 
the performance benefit.
> 
> I think the fix is for user space to set SSBS in the child if it no
> longer needs it.
> 

Sorry for the late response on this.

Thanks,
-- Srinivas R


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