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

[Srinivas Ramana <sramana@codeaurora.org>]

On 12/24/2019 12:36 PM, Anshuman Khandual wrote:
> 
> 
> 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 ?
> 
Yes, that is correct.

>>
>> Fixes: 8f04e8e6e29c ("arm64: ssbd: Add support for PSTATE.SSBS rather than trapping to EL3")
>> Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>
>> ---
>>   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;
>>

Thanks,
-- Srinivas R

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