[PATCH v2] powerpc: select ARCH_HAS_MEMBARRIER_SYNC_CORE

[Nicholas Piggin]

Excerpts from Mathieu Desnoyers's message of July 15, 2020 10:27 pm:
> ----- On Jul 15, 2020, at 5:48 AM, Nicholas Piggin npiggin at gmail.com wrote:
> [...]
>> index 47bd4ea0837d..a4704f405e8d 100644
>> --- a/arch/powerpc/include/asm/exception-64s.h
>> +++ b/arch/powerpc/include/asm/exception-64s.h
>> @@ -68,6 +68,13 @@
>>  *
>>  * The nop instructions allow us to insert one or more instructions to flush the
>>  * L1-D cache when returning to userspace or a guest.
>> + *
>> + * powerpc relies on return from interrupt/syscall being context synchronising
>> + * (which hrfid, rfid, and rfscv are) to support ARCH_HAS_MEMBARRIER_SYNC_CORE
>> + * without additional additional synchronisation instructions. soft-masked
>> + * interrupt replay does not include a context-synchronising rfid, but those
>> + * always return to kernel, the context sync is only required for IPIs which
>> + * return to user.
>>  */
>> #define RFI_FLUSH_SLOT							\
>> 	RFI_FLUSH_FIXUP_SECTION;					\
> 
> I suspect the statement "the context sync is only required for IPIs which return to
> user." is misleading.
> 
> As I recall that we need more than just context sync after IPI. We need context sync
> in return path of any trap/interrupt/system call which returns to user-space, else
> we'd need to add the proper core serializing barriers in the scheduler, as we had
> to do for lazy tlb on x86.
> 
> Or am I missing something ?

Maybe ambiguous wording. For IPIs, the context synch is only required 
for those which return to user. Other things also require context sync.

I will try to improve it.

Thanks,
Nick