Re: [Qemu-devel] [PATCH RFC 2/2] vfio-ccw: support for halt/clear subchannel

[Pierre Morel <pmorel@linux.ibm.com>]

On 15/05/2018 18:10, Cornelia Huck wrote:
> On Fri, 11 May 2018 11:33:35 +0200
> Pierre Morel <pmorel@linux.ibm.com> wrote:
>
>> On 09/05/2018 17:48, Cornelia Huck wrote:
>>> Currently, vfio-ccw only relays start subchannel requests to the real
>>> hardware, which is enough in many cases but falls short e.g. during
>>> error recovery.
>>>
>>> Fortunately, it is easy to add support for halt and clear subchannel
>>> requests to the existing infrastructure. User space can detect
>>> support for halt/clear subchannel easily, as we always returned
>>> -EOPNOTSUPP before and therefore we do not need any capability to
>>> make this support discoverable.
>>>
>>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>> ---
>>>    drivers/s390/cio/vfio_ccw_drv.c | 10 ++++-
>>>    drivers/s390/cio/vfio_ccw_fsm.c | 94 ++++++++++++++++++++++++++++++++++++-----
>>>    2 files changed, 92 insertions(+), 12 deletions(-)
>>> @@ -65,6 +67,70 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
>>>    	return ret;
>>>    }
>>>    
>>> +static int fsm_halt_helper(struct vfio_ccw_private *private)
>>> +{
>>> +	struct subchannel *sch;
>>> +	int ccode;
>>> +	unsigned long flags;
>>> +	int ret;
>>> +
>>> +	sch = private->sch;
>>> +
>>> +	spin_lock_irqsave(sch->lock, flags);
>>> +	private->state = VFIO_CCW_STATE_BUSY;
>>> +
>>> +	/* Issue "Halt Subchannel" */
>>> +	ccode = hsch(sch->schid);
>>> +
>>> +	switch (ccode) {
>>> +	case 0:
>>> +		/*
>>> +		 * Initialize device status information
>>> +		 */
>>> +		sch->schib.scsw.cmd.actl |= SCSW_ACTL_HALT_PEND;
>>> +		ret = 0;
>>> +		break;
>>> +	case 1:		/* Status pending */
>> shouldn't we make a difference between status pending
>> and having halt in progress?
>>
>> The guest can examine the SCSW, but couldn't it introduce
>> a race condition?
> Yes, good point. Especially as the guest might want to do different
> things.
>
> Regarding race conditions: The scsw can already be outdated after the
> operation that stored it finished, which is true even on LPAR. That's
> especially true for tsch which clears some status at the subchannel.
> The guest must already be able to deal with this, the race window is
> just larger.

This is the kind of race I try to avoid with the mutex protected
state changes patch.

>
>>
>>> +	case 2:		/* Busy */
>>> +		ret = -EBUSY;
>>> +		break;
>>> +	default:	/* Device not operational */
>>> +		ret = -ENODEV;
>>> +	}
>>> +	spin_unlock_irqrestore(sch->lock, flags);
>>> +	return ret;
>>> +}
>>> +
>>> +static int fsm_clear_helper(struct vfio_ccw_private *private)
>>> +{
>>> +	struct subchannel *sch;
>>> +	int ccode;
>>> +	unsigned long flags;
>>> +	int ret;
>>> +
>>> +	sch = private->sch;
>>> +
>>> +	spin_lock_irqsave(sch->lock, flags);
>>> +	private->state = VFIO_CCW_STATE_BUSY;
>>> +
>>> +	/* Issue "Clear Subchannel" */
>>> +	ccode = csch(sch->schid);
>>> +
>>> +	switch (ccode) {
>>> +	case 0:
>>> +		/*
>>> +		 * Initialize device status information
>>> +		 */
>>> +		sch->schib.scsw.cmd.actl |= SCSW_ACTL_CLEAR_PEND;
>>> +		ret = 0;
>>> +		break;
>>> +	default:	/* Device not operational */
>>> +		ret = -ENODEV;
>>> +	}
>>> +	spin_unlock_irqrestore(sch->lock, flags);
>>> +	return ret;
>>> +}
>>> +
>>>    static void fsm_notoper(struct vfio_ccw_private *private,
>>>    			enum vfio_ccw_event event)
>>>    {
>>> @@ -126,7 +192,24 @@ static void fsm_io_request(struct vfio_ccw_private *private,
>>>    
>>>    	memcpy(scsw, io_region->scsw_area, sizeof(*scsw));
>>>    
>>> -	if (scsw->cmd.fctl & SCSW_FCTL_START_FUNC) {
>>> +	/*
>>> +	 * Start processing with the clear function, then halt, then start.
>>> +	 * We may still be start pending when the caller wants to clean
>>> +	 * up things via halt/clear.
>>> +	 */
>> hum. The scsw here does not reflect the hardware state but the
>> command passed from the user interface.
>> Can we and should we authorize multiple commands in one call?
>>
>> If not, the comment is not appropriate and a switch on cmd.fctl
>> would be a clearer.
> There may be multiple functions specified, but we need to process them
> in precedence order (and clear wins over the others, so to speak).
> Would adding a sentence like "we always process just one function" help?

Why should we allow multiple commands in a single call ?
It brings no added value.
Is there a use case?
Currently QEMU does not do this and since we only have the SCSH there
is no difference having the bit set alone or not alone.


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany