On 06/07/2017 08:29 PM, Noam Camus wrote: > *From:* Noam Camus > *Sent:* Wednesday, June 7, 2017 8:06:17 PM > *To:* Vineet Gupta; linux-snps-arc at lists.infradead.org > *Cc:* linux-kernel at vger.kernel.org; Elad Kanfi > *Subject:* Re: [PATCH v2 11/11] ARC: [plat-eznps] Handle memory error as an > exception > > *> From:*Vineet Gupta <Vineet.Gupta1 at synopsys.com> > > *> Sent:* Wednesday, June 7, 2017 7:15 PM... > > > So NPS *hardware* generates exception, jumps to vector mem_service(), which you > > redirect to the machine check handler - which simply panics. > > But this redirection is under EZNPS_MEM_ERROR, which you have defaulted to > "n". So > > how is the default working for hardware ? Doesn't it need to be "y" > > The NPS400 architects changed userspace bus error behavior to be machine check > instead of Interrupt level 2. > The reason was that since we are dealing with imprecise exception. > So memory request result will be back to core long time after bad instruction > was executed. > In the meantime core be able to do HW schedule between threads and result may > hit another thread. > The core do not keep information on each such bus transaction so it just > interfere current thread without knowing if it was the initiator of this bus > transaction. > In such case we prefer to create machine check and end with PANIC. Ok this make sense ! > > With simulator we just turn this configuration on, so we redirect the Legacy > Synopsys L2 ISR from nSIM into machine check. > This way we end up just like with silicon ? This doesn't make sense :-) In simulation (where L2 interrupt is asserted), you need to handle it as such - say reading out the banked regs for L2 interrupt. What you are doing here is handling it like an exception which won't work . I really don't see the point of this "alignment" - hardware and simulation are different. simulation semantics are already supported by generic ARC code. And for silicon case, the existing vector woudl MachineCheck would work for both K and U. So I'm not sure what we are trying to achieve here ! > > > >BTW it seems your patch is wrong otherwise too. So the userspace bus error will go > >to machine check handler which currently just panic's. You really want to kill the > >user space process and continue, thus need to call do_memory_error() > So I believe that we do correct thing here, when we deal with multi thread cores. Sure, the imprecise handling of bus error is an issue - but we should atleat try to recover. By just panic'ing unconditionally, you are enabling a one liner user program to panic the system (granted in simulation only) -Vineet
