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

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Date:   Thu, 9 Jan 2020 15:17:57 +0000
From:   Will Deacon <will@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Srinivas Ramana <sramana@...eaurora.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 Thu, Jan 02, 2020 at 06:01:45PM +0000, 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
> 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().

Right, I'd expect the parent SSBS to be inherited when we copy the pstate
field along with the other regs, and I think this is the correct behaviour.

Is that broken somehow?

Will