Re: [Qemu-devel] [PATCH 2/9] s390x: fix invalid use of cc 1 for SSCH

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 08/31/2017 09:50 AM, Thomas Huth wrote:
> On 30.08.2017 18:36, Halil Pasic wrote:
>> According to the POP a start subchannel instruction (SSCH) returning with
>> cc 1 implies that the subchannel was status pending when SSCH executed.
>>
>> Due to a somewhat confusing error handling, where error codes are mapped
>> to cc value, sane looking error codes result in non AR compliant
>> behavior.
>>
>> Let's fix this! Instead of cc 1 we use cc 3 which means device not
>> operational, and is much closer to the truth in the given cases.
>>
>> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
>> Acked-by: Pierre Morel<pmorel@linux.vnet.ibm.com>
>> ---
>>
>> This patch turned out quite controversial. We did not reach a consensus
>> during the internal review.
>>
>> The most of the discussion revolved around the ORB flag which
>> architecturally must be supported, but are currently not supported by
>> vfio-ccw (not yet, or can't be). The idea showing the most promise for
>> consensus was to handle this via device status (along the lines better a
>> strange acting device than a non-conform machine) but since it's a
>> radical change we decided to first discuss upstream and then do whatever
>> needs to be done.
>> ---
>>  hw/s390x/css.c      | 15 ++++++---------
>>  hw/s390x/s390-ccw.c |  2 +-
>>  2 files changed, 7 insertions(+), 10 deletions(-)
>>
>> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
>> index a50fb0727e..0822538cde 100644
>> --- a/hw/s390x/css.c
>> +++ b/hw/s390x/css.c
>> @@ -1034,7 +1034,7 @@ static int sch_handle_start_func_passthrough(SubchDev *sch)
>>       */
>>      if (!(orb->ctrl0 & ORB_CTRL0_MASK_PFCH) ||
>>          !(orb->ctrl0 & ORB_CTRL0_MASK_C64)) {
>> -        return -EINVAL;
>> +        return -ENODEV;
> 
> I don't really like ENODEV in this case (since the device is apparently
> there)... but well, since you're later change it again to set cc=3
> directly, I guess the temporary ENODEV is ok.
> 
>>      }
>>  
>>      ret = s390_ccw_cmd_request(orb, s, sch->driver_data);
>> @@ -1046,16 +1046,13 @@ static int sch_handle_start_func_passthrough(SubchDev *sch)
>>          break;
>>      case -ENODEV:
>>          break;
>> +    case -EFAULT:
>> +         break;
> 
> I think you should mention this in the patch description. Why is EFAULT
> suddenly handled here?

It is not suddenly :) If you examine ioinst_handle_ssch which really
handles the error codes (here we are just mapping them around) you see:
   switch (ret) {
    case -ENODEV:
        cc = 3;
        break;
    case -EBUSY:
        cc = 2;
        break;
    case -EFAULT:
        /*
         * TODO:
         * I'm wondering whether there is something better
         * to do for us here (like setting some device or
         * subchannel status).
         */
        program_interrupt(env, PGM_ADDRESSING, 4);
        return;
    case 0:
        cc = 0;
        break;
    default:
        cc = 1;
        break;
    }

That is -EFAULT is handled with a program interrupt, and I want to keep
that. Hence break, that is keep unchanged.

What I do want to change is the other not explicitly handled error codes
(which actually should not happen) should be cc 3 and not cc 1.

So the default branch sets ret to -ENODEV.


> 
>>      case -EACCES:
>>          /* Let's reflect an inaccessible host device by cc 3. */
>> -        ret = -ENODEV;
>> -        break;
>>      default:
>> -       /*
>> -        * All other return codes will trigger a program check,
>> -        * or set cc to 1.
>> -        */
>> -       break;
>> +        /* Let's make all other return codes map to cc 3.  */
>> +        ret = -ENODEV;
>>      };
>>  
>>      return ret;
>> @@ -1115,7 +1112,7 @@ static int do_subchannel_work(SubchDev *sch)
>>      if (sch->do_subchannel_work) {
>>          return sch->do_subchannel_work(sch);
>>      } else {
>> -        return -EINVAL;
>> +        return -ENODEV;
>>      }
>>  }
>>  
>> diff --git a/hw/s390x/s390-ccw.c b/hw/s390x/s390-ccw.c
>> index 8614dda6f8..2b0741741c 100644
>> --- a/hw/s390x/s390-ccw.c
>> +++ b/hw/s390x/s390-ccw.c
>> @@ -25,7 +25,7 @@ int s390_ccw_cmd_request(ORB *orb, SCSW *scsw, void *data)
>>      if (cdc->handle_request) {
>>          return cdc->handle_request(orb, scsw, data);
>>      } else {
>> -        return -ENOSYS;
>> +        return -ENODEV;
>>      }
>>  }
> 
>  Thomas
>