From: Eric Farman <firstname.lastname@example.org> To: Cornelia Huck <email@example.com> Cc: Jared Rossi <firstname.lastname@example.org>, Halil Pasic <email@example.com>, firstname.lastname@example.org, email@example.com, Eric Farman <firstname.lastname@example.org> Subject: [RFC PATCH v2 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR Date: Wed, 13 May 2020 16:29:30 +0200 [thread overview] Message-ID: <email@example.com> (raw) Hi Conny, Back in January, I suggested a small patch  to try to clean up the handling of HSCH/CSCH interrupts, especially as it relates to concurrent SSCH interrupts. Here is a new attempt to address this. There was some suggestion earlier about locking the FSM, but I'm not seeing any problems with that. Rather, what I'm noticing is that the flow between a synchronous START and asynchronous HALT/CLEAR have different impacts on the FSM state. Consider: CPU 1 CPU 2 SSCH (set state=CP_PENDING) INTERRUPT (set state=IDLE) CSCH (no change in state) SSCH (set state=CP_PENDING) INTERRUPT (set state=IDLE) INTERRUPT (set state=IDLE) The second interrupt on CPU 1 will call cp_free() for the START created by CPU 2, and our results will be, um, messy. This suggests that three things must be true: A) cp_free() must be called for either a final interrupt, or a failure issuing a SSCH B) The FSM state must only be set to IDLE once cp_free() has been called C) A SSCH cannot be issued while an interrupt is outstanding It's not great that items B and C are separated in the interrupt path by a mutex boundary where the copy into io_region occurs. We could (and perhaps should) move them together, which would improve things somewhat, but still doesn't address the scenario laid out above. Even putting that work within the mutex window during interrupt processing doesn't address things totally. So, the patches here do two things. One to handle unsolicited interrupts , which I think is pretty straightforward. Though, it does make me want to do a more drastic rearranging of the affected function. :) The other warrants some discussion, since it's sort of weird. Basically, recognize what commands we've issued, expect interrupts for each, and prevent new SSCH's while asynchronous commands are pending. It doesn't address interrupts from the HSCH/CSCH that could be generated by the Channel Path Handling code  for an offline CHPID yet, and it needs some TLC to make checkpatch happy. But it's the best possibility I've seen thus far. I used private->scsw for this because it's barely used for anything else; at one point I had been considering removing it outright because we have entirely too many SCSW's in this code. :) I could implement this as three small flags in private, to simplify the code and avoid confusion with the different fctl/actl flags in private vs schib. It does make me wonder whether the CP_PENDING check before cp_free() is still necessary, but that's a query for a later date. Also, perhaps this could be accomplished with two new FSM states, one for a HALT and one for a CLEAR. I put it aside because the interrupt handler got a little hairy with that, but I'm still looking at it in parallel to this discussion. I look forward to hearing your thoughts...  https://firstname.lastname@example.org/  https://email@example.com/  https://firstname.lastname@example.org/ Eric Farman (4): vfio-ccw: Do not reset FSM state for unsolicited interrupts vfio-ccw: Utilize scsw actl to serialize start operations vfio-ccw: Expand SCSW usage to HALT and CLEAR vfio-ccw: Clean up how to react to a failed START drivers/s390/cio/vfio_ccw_drv.c | 10 +++++++++- drivers/s390/cio/vfio_ccw_fsm.c | 26 ++++++++++++++++++++++++++ drivers/s390/cio/vfio_ccw_ops.c | 3 +-- 3 files changed, 36 insertions(+), 3 deletions(-) -- 2.17.1
next reply other threads:[~2020-05-13 14:29 UTC|newest] Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-05-13 14:29 Eric Farman [this message] 2020-05-13 14:29 ` [RFC PATCH v2 1/4] vfio-ccw: Do not reset FSM state for unsolicited interrupts Eric Farman 2020-05-13 14:29 ` [RFC PATCH v2 2/4] vfio-ccw: Utilize scsw actl to serialize start operations Eric Farman 2020-05-13 14:29 ` [RFC PATCH v2 3/4] vfio-ccw: Expand SCSW usage to HALT and CLEAR Eric Farman 2020-05-13 14:29 ` [RFC PATCH v2 4/4] vfio-ccw: Clean up how to react to a failed START Eric Farman 2020-05-14 13:46 ` [RFC PATCH v2 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR Halil Pasic 2020-05-15 13:09 ` Eric Farman 2020-05-15 14:55 ` Halil Pasic 2020-05-15 15:58 ` Cornelia Huck 2020-05-15 17:41 ` Halil Pasic 2020-05-15 18:19 ` Eric Farman 2020-05-15 18:12 ` Eric Farman 2020-05-15 18:37 ` Halil Pasic 2020-05-18 22:01 ` Eric Farman 2020-05-15 19:35 ` Halil Pasic 2020-05-18 16:09 ` Cornelia Huck 2020-05-18 21:57 ` Eric Farman 2020-05-19 11:23 ` Cornelia Huck 2020-05-18 22:09 ` Halil Pasic 2020-05-19 11:36 ` Cornelia Huck 2020-05-19 12:10 ` Halil Pasic 2020-05-26 9:55 ` Cornelia Huck 2020-05-26 11:08 ` Eric Farman
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