Re: [Qemu-devel] [PATCH v3 2/6] vfio-ccw: rework ssch state handling

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 5 Feb 2019 09:31:55 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> On 02/05/2019 07:10 AM, Cornelia Huck wrote:
> > On Mon, 4 Feb 2019 16:29:40 -0500
> > Eric Farman <farman@linux.ibm.com> wrote:
> >   
> >> On 01/30/2019 08:22 AM, Cornelia Huck wrote:  
> >>> The flow for processing ssch requests can be improved by splitting
> >>> the BUSY state:
> >>>
> >>> - CP_PROCESSING: We reject any user space requests while we are in
> >>>     the process of translating a channel program and submitting it to
> >>>     the hardware. Use -EAGAIN to signal user space that it should
> >>>     retry the request.
> >>> - CP_PENDING: We have successfully submitted a request with ssch and
> >>>     are now expecting an interrupt. As we can't handle more than one
> >>>     channel program being processed, reject any further requests with
> >>>     -EBUSY. A final interrupt will move us out of this state; this also
> >>>     fixes a latent bug where a non-final interrupt might have freed up
> >>>     a channel program that still was in progress.
> >>>     By making this a separate state, we make it possible to issue a
> >>>     halt or a clear while we're still waiting for the final interrupt
> >>>     for the ssch (in a follow-on patch).
> >>>
> >>> It also makes a lot of sense not to preemptively filter out writes to
> >>> the io_region if we're in an incorrect state: the state machine will
> >>> handle this correctly.
> >>>
> >>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> >>> ---
> >>>    drivers/s390/cio/vfio_ccw_drv.c     |  8 ++++++--
> >>>    drivers/s390/cio/vfio_ccw_fsm.c     | 19 ++++++++++++++-----
> >>>    drivers/s390/cio/vfio_ccw_ops.c     |  2 --
> >>>    drivers/s390/cio/vfio_ccw_private.h |  3 ++-
> >>>    4 files changed, 22 insertions(+), 10 deletions(-)  
> >   
> >>> diff --git a/drivers/s390/cio/vfio_ccw_fsm.c b/drivers/s390/cio/vfio_ccw_fsm.c
> >>> index e7c9877c9f1e..b4a141fbd1a8 100644
> >>> --- a/drivers/s390/cio/vfio_ccw_fsm.c
> >>> +++ b/drivers/s390/cio/vfio_ccw_fsm.c
> >>> @@ -28,7 +28,6 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
> >>>    	sch = private->sch;
> >>>    
> >>>    	spin_lock_irqsave(sch->lock, flags);
> >>> -	private->state = VFIO_CCW_STATE_BUSY;
> >>>    
> >>>    	orb = cp_get_orb(&private->cp, (u32)(addr_t)sch, sch->lpm);
> >>>    	if (!orb) {
> >>> @@ -46,6 +45,7 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
> >>>    		 */
> >>>    		sch->schib.scsw.cmd.actl |= SCSW_ACTL_START_PEND;
> >>>    		ret = 0;
> >>> +		private->state = VFIO_CCW_STATE_CP_PENDING;  
> >>
> >> [1]
> >>  
> >>>    		break;
> >>>    	case 1:		/* Status pending */
> >>>    	case 2:		/* Busy */
> >>> @@ -107,6 +107,12 @@ static void fsm_io_busy(struct vfio_ccw_private *private,
> >>>    	private->io_region->ret_code = -EBUSY;
> >>>    }
> >>>    
> >>> +static void fsm_io_retry(struct vfio_ccw_private *private,
> >>> +			 enum vfio_ccw_event event)
> >>> +{
> >>> +	private->io_region->ret_code = -EAGAIN;
> >>> +}
> >>> +
> >>>    static void fsm_disabled_irq(struct vfio_ccw_private *private,
> >>>    			     enum vfio_ccw_event event)
> >>>    {
> >>> @@ -135,8 +141,7 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> >>>    	struct mdev_device *mdev = private->mdev;
> >>>    	char *errstr = "request";
> >>>    
> >>> -	private->state = VFIO_CCW_STATE_BUSY;
> >>> -
> >>> +	private->state = VFIO_CCW_STATE_CP_PROCESSING;  
> >>
> >> [1]
> >>  
> >>>    	memcpy(scsw, io_region->scsw_area, sizeof(*scsw));
> >>>    
> >>>    	if (scsw->cmd.fctl & SCSW_FCTL_START_FUNC) {
> >>> @@ -181,7 +186,6 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> >>>    	}
> >>>    
> >>>    err_out:
> >>> -	private->state = VFIO_CCW_STATE_IDLE;  
> >>
> >> [1] Revisiting these locations as from an earlier discussion [2]...
> >> These go IDLE->CP_PROCESSING->CP_PENDING if we get a cc=0 on the SSCH,
> >> but we stop in CP_PROCESSING if the SSCH gets a nonzero cc.  Shouldn't
> >> we cleanup and go back to IDLE in this scenario, rather than forcing
> >> userspace to escalate to CSCH/HSCH after some number of retries (via FSM)?
> >>
> >> [2] https://patchwork.kernel.org/patch/10773611/#22447997  
> > 
> > It does do that (in vfio_ccw_mdev_write), it was not needed here. Or do
> > you think doing it here would be more obvious?  
> 
> Ah, my mistake, I missed that.  (That function is renamed to 
> vfio_ccw_mdev_write_io_region in patch 4.)
> 
> I don't think keeping it here is necessary then.  I got too focused 
> looking at what you ripped out that I lost the things that stayed.  Once 
> this series gets in its entirety, and Pierre has a chance to rebase his 
> FSM series on top of it all, this should be in great shape.

Yeah, it's probably easier to look at the end result.

> 
> >   
> >>
> >> Besides that, I think this looks good to me.  
> > 
> > Thanks!
> >   
> 
> You're welcome!  Here, have a thing to add to this patch:
> 
> Reviewed-by: Eric Farman <farman@linux.ibm.com>
> 

Thanks a lot!