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

[Anshuman Khandual]

On 12/23/2019 06:32 PM, 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.

So this fixes the situation (i.e low performance) from the creation time
of a task with fork() which will never see a subsequent execve, till it
gets context switched for the very first time ?

> 
> Fixes: 8f04e8e6e29c ("arm64: ssbd: Add support for PSTATE.SSBS rather than trapping to EL3")
> Signed-off-by: Srinivas Ramana <sramana@xxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/process.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 71f788cd2b18..a8f05cc39261 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -399,6 +399,13 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
>  		 */
>  		if (clone_flags & CLONE_SETTLS)
>  			p->thread.uw.tp_value = childregs->regs[3];
> +
> +		if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE) {
> +			if (is_compat_thread(task_thread_info(p)))
> +				set_compat_ssbs_bit(childregs);
> +			else
> +				set_ssbs_bit(childregs);
> +		}
>  	} else {
>  		memset(childregs, 0, sizeof(struct pt_regs));
>  		childregs->pstate = PSR_MODE_EL1h;
>