Re: [PATCH v2] s390x/css: revert SCSW ctrl/flag bits on error

[Matthew Rosato]

On 10/28/22 4:22 PM, Eric Farman wrote:
> On Thu, 2022-10-27 at 23:23 +0200, Peter Jin wrote:
>> Revert the control and flag bits in the subchannel status word in
>> case
>> the SSCH operation fails with non-zero CC (ditto for CSCH and HSCH).
>> According to POPS, the control and flag bits are only changed if
>> SSCH,
>> CSCH, and HSCH return CC 0, and no other action should be taken
>> otherwise.
>> In order to simulate that after the fact, the bits need to be
>> reverted on
>> non-zero CC.
>>
> 
> I'm okay to this point...
> 
>> This change is necessary due to the fact that the pwrite() in vfio-
>> ccw
>> which triggers the SSCH can fail at any time. Previously, there was
>> only virtio-ccw, whose do_subchannel_work function was only able to
>> return CC0. However, once vfio-ccw went into the mix, it has become
>> necessary to handle errors in code paths that were previously assumed
>> to always return success.
>>
>> In our case, we found that in case of pwrite() failure (which was
>> discovered by strace injection), the subchannel could be stuck in
>> start
>> pending state, which could be problematic if the pwrite() call
>> returns
>> CC2. Experimentation shows that the guest tries to retry the SSCH
>> call as
>> normal for CC2, but it actually continously fails due to the fact
>> that
>> the subchannel is stuck in start pending state even though no start
>> function is actually taking place.
> 
> ...but the two paragraphs above are a bit cumbersome to digest. Maybe
> it's just too late in the week for me. What about something like this?
> 
> """
> While the do_subchannel_work logic for virtual (virtio) devices will
> return condition code 0, passthrough (vfio) devices may encounter
> errors from either the host kernel or real hardware that need to be
> accounted for after this point. This includes restoring the state of
> the Subchannel Status Word to reflect the subchannel, as these bits
> would not be set in the event of a non-zero condition code from the
> affected instructions.
> 
> Experimentation has shown that a failure on a START SUBCHANNEL (SSCH)
> to a passthrough device would leave the subchannel with the START
> PENDING activity control bit set, thus blocking subsequent SSCH
> operations in css_do_ssch() until some form of error recovery was
> undertaken since no interrupt would be expected.
> """

+1 to this re-write

> 
>>
>> Signed-off-by: Peter Jin <pjin@linux.ibm.com>
> 
> We've talked previously about clearing this within the
> do_subchannel_work_passthrough routine in order to keep the _virtual
> paths untouched, but this seems like a reasonable approach to me.
> 
> The commit message is probably fine either way, but as far as the code
> goes:
> 
> Reviewed-by: Eric Farman <farman@linux.ibm.com>

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

Thanks Peter!

> 
>> ---
>>  hw/s390x/css.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++-
>> --
>>  1 file changed, 48 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
>> index 7d9523f811..95d1b3a3ce 100644
>> --- a/hw/s390x/css.c
>> +++ b/hw/s390x/css.c
>> @@ -1522,21 +1522,37 @@ IOInstEnding css_do_xsch(SubchDev *sch)
>>  IOInstEnding css_do_csch(SubchDev *sch)
>>  {
>>      SCHIB *schib = &sch->curr_status;
>> +    uint16_t old_scsw_ctrl;
>> +    IOInstEnding ccode;
>>  
>>      if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
>> PMCW_FLAGS_MASK_ENA)) {
>>          return IOINST_CC_NOT_OPERATIONAL;
>>      }
>>  
>> +    /*
>> +     * Save the current scsw.ctrl in case CSCH fails and we need
>> +     * to revert the scsw to the status quo ante.
>> +     */
>> +    old_scsw_ctrl = schib->scsw.ctrl;
>> +
>>      /* Trigger the clear function. */
>>      schib->scsw.ctrl &= ~(SCSW_CTRL_MASK_FCTL |
>> SCSW_CTRL_MASK_ACTL);
>>      schib->scsw.ctrl |= SCSW_FCTL_CLEAR_FUNC | SCSW_ACTL_CLEAR_PEND;
>>  
>> -    return do_subchannel_work(sch);
>> +    ccode = do_subchannel_work(sch);
>> +
>> +    if (ccode != IOINST_CC_EXPECTED) {
>> +        schib->scsw.ctrl = old_scsw_ctrl;
>> +    }
>> +
>> +    return ccode;
>>  }
>>  
>>  IOInstEnding css_do_hsch(SubchDev *sch)
>>  {
>>      SCHIB *schib = &sch->curr_status;
>> +    uint16_t old_scsw_ctrl;
>> +    IOInstEnding ccode;
>>  
>>      if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
>> PMCW_FLAGS_MASK_ENA)) {
>>          return IOINST_CC_NOT_OPERATIONAL;
>> @@ -1553,6 +1569,12 @@ IOInstEnding css_do_hsch(SubchDev *sch)
>>          return IOINST_CC_BUSY;
>>      }
>>  
>> +    /*
>> +     * Save the current scsw.ctrl in case HSCH fails and we need
>> +     * to revert the scsw to the status quo ante.
>> +     */
>> +    old_scsw_ctrl = schib->scsw.ctrl;
>> +
>>      /* Trigger the halt function. */
>>      schib->scsw.ctrl |= SCSW_FCTL_HALT_FUNC;
>>      schib->scsw.ctrl &= ~SCSW_FCTL_START_FUNC;
>> @@ -1564,7 +1586,13 @@ IOInstEnding css_do_hsch(SubchDev *sch)
>>      }
>>      schib->scsw.ctrl |= SCSW_ACTL_HALT_PEND;
>>  
>> -    return do_subchannel_work(sch);
>> +    ccode = do_subchannel_work(sch);
>> +
>> +    if (ccode != IOINST_CC_EXPECTED) {
>> +        schib->scsw.ctrl = old_scsw_ctrl;
>> +    }
>> +
>> +    return ccode;
>>  }
>>  
>>  static void css_update_chnmon(SubchDev *sch)
>> @@ -1605,6 +1633,8 @@ static void css_update_chnmon(SubchDev *sch)
>>  IOInstEnding css_do_ssch(SubchDev *sch, ORB *orb)
>>  {
>>      SCHIB *schib = &sch->curr_status;
>> +    uint16_t old_scsw_ctrl, old_scsw_flags;
>> +    IOInstEnding ccode;
>>  
>>      if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
>> PMCW_FLAGS_MASK_ENA)) {
>>          return IOINST_CC_NOT_OPERATIONAL;
>> @@ -1626,11 +1656,26 @@ IOInstEnding css_do_ssch(SubchDev *sch, ORB
>> *orb)
>>      }
>>      sch->orb = *orb;
>>      sch->channel_prog = orb->cpa;
>> +
>> +    /*
>> +     * Save the current scsw.ctrl and scsw.flags in case SSCH fails
>> and we need
>> +     * to revert the scsw to the status quo ante.
>> +     */
>> +    old_scsw_ctrl = schib->scsw.ctrl;
>> +    old_scsw_flags = schib->scsw.flags;
>> +
>>      /* Trigger the start function. */
>>      schib->scsw.ctrl |= (SCSW_FCTL_START_FUNC |
>> SCSW_ACTL_START_PEND);
>>      schib->scsw.flags &= ~SCSW_FLAGS_MASK_PNO;
>>  
>> -    return do_subchannel_work(sch);
>> +    ccode = do_subchannel_work(sch);
>> +
>> +    if (ccode != IOINST_CC_EXPECTED) {
>> +        schib->scsw.ctrl = old_scsw_ctrl;
>> +        schib->scsw.flags = old_scsw_flags;
>> +    }
>> +
>> +    return ccode;
>>  }
>>  
>>  static void copy_irb_to_guest(IRB *dest, const IRB *src, const PMCW
>> *pmcw,
>