Re: [PATCH 1/7] s390/cio: Update SCSW if it points to the end of the chain

[Eric Farman <farman@linux.ibm.com>]

On 5/6/19 10:47 AM, Cornelia Huck wrote:
> On Fri,  3 May 2019 15:49:06 +0200
> Eric Farman <farman@linux.ibm.com> wrote:
> 
>> Per the POPs [1], when processing an interrupt the SCSW.CPA field of an
>> IRB generally points to 8 bytes after the last CCW that was executed
>> (there are exceptions, but this is the most common behavior).
>>
>> In the case of an error, this points us to the first un-executed CCW
>> in the chain.  But in the case of normal I/O, the address points beyond
>> the end of the chain.  While the guest generally only cares about this
>> when possibly restarting a channel program after error recovery, we
>> should convert the address even in the good scenario so that we provide
>> a consistent, valid, response upon I/O completion.
>>
>> [1] Figure 16-6 in SA22-7832-11.  The footnotes in that table also state
>> that this is true even if the resulting address is invalid or protected,
>> but moving to the end of the guest chain should not be a surprise.
>>
>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>> ---
>>   drivers/s390/cio/vfio_ccw_cp.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/s390/cio/vfio_ccw_cp.c b/drivers/s390/cio/vfio_ccw_cp.c
>> index 384b3987eeb4..f86da78eaeaa 100644
>> --- a/drivers/s390/cio/vfio_ccw_cp.c
>> +++ b/drivers/s390/cio/vfio_ccw_cp.c
>> @@ -870,7 +870,7 @@ void cp_update_scsw(struct channel_program *cp, union scsw *scsw)
>>   	 */
>>   	list_for_each_entry(chain, &cp->ccwchain_list, next) {
>>   		ccw_head = (u32)(u64)chain->ch_ccw;
>> -		if (is_cpa_within_range(cpa, ccw_head, chain->ch_len)) {
>> +		if (is_cpa_within_range(cpa, ccw_head, chain->ch_len + 1)) {
> 
> Maybe add a comment
> 
> /* On successful execution, cpa points just beyond the end of the chain. */
> 
> or so, to avoid head-scratching and PoP-reading in the future?

And deny future visitors the confusion?  :)

Good point; added.

> 
>>   			/*
>>   			 * (cpa - ccw_head) is the offset value of the host
>>   			 * physical ccw to its chain head.
>