Re: [PATCH v6 3/3] vfio-ccw: Serialize FSM IDLE state with I/O completion

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 5/11/21 3:56 PM, Eric Farman wrote:
> Today, the stacked call to vfio_ccw_sch_io_todo() does three things:
> 
>    1) Update a solicited IRB with CP information, and release the CP
>       if the interrupt was the end of a START operation.
>    2) Copy the IRB data into the io_region, under the protection of
>       the io_mutex
>    3) Reset the vfio-ccw FSM state to IDLE to acknowledge that
>       vfio-ccw can accept more work.
> 
> The trouble is that step 3 is (A) invoked for both solicited and
> unsolicited interrupts, and (B) sitting after the mutex for step 2.
> This second piece becomes a problem if it processes an interrupt
> for a CLEAR SUBCHANNEL while another thread initiates a START,
> thus allowing the CP and FSM states to get out of sync. That is:
> 
>      CPU 1                           CPU 2
>      fsm_do_clear()
>      fsm_irq()
>                                      fsm_io_request()
>      vfio_ccw_sch_io_todo()
>                                      fsm_io_helper()
> 
> Since the FSM state and CP should be kept in sync, let's make a
> note when the CP is released, and rely on that as an indication
> that the FSM should also be reset at the end of this routine and
> open up the device for more work.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>

Thanks for the detailed commit message and comment block -- this makes 
sense to me.

Acked-by: Matthew Rosato <mjrosato@linux.ibm.com>

> ---
>   drivers/s390/cio/vfio_ccw_drv.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
> index 8c625b530035..9b61e9b131ad 100644
> --- a/drivers/s390/cio/vfio_ccw_drv.c
> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> @@ -86,6 +86,7 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
>   	struct vfio_ccw_private *private;
>   	struct irb *irb;
>   	bool is_final;
> +	bool cp_is_finished = false;
>   
>   	private = container_of(work, struct vfio_ccw_private, io_work);
>   	irb = &private->irb;
> @@ -94,14 +95,21 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
>   		     (SCSW_ACTL_DEVACT | SCSW_ACTL_SCHACT));
>   	if (scsw_is_solicited(&irb->scsw)) {
>   		cp_update_scsw(&private->cp, &irb->scsw);
> -		if (is_final && private->state == VFIO_CCW_STATE_CP_PENDING)
> +		if (is_final && private->state == VFIO_CCW_STATE_CP_PENDING) {
>   			cp_free(&private->cp);
> +			cp_is_finished = true;
> +		}
>   	}
>   	mutex_lock(&private->io_mutex);
>   	memcpy(private->io_region->irb_area, irb, sizeof(*irb));
>   	mutex_unlock(&private->io_mutex);
>   
> -	if (private->mdev && is_final)
> +	/*
> +	 * Reset to IDLE only if processing of a channel program
> +	 * has finished. Do not overwrite a possible processing
> +	 * state if the final interrupt was for HSCH or CSCH.
> +	 */
> +	if (private->mdev && cp_is_finished)
>   		private->state = VFIO_CCW_STATE_IDLE;
>   
>   	if (private->io_trigger)
>