Re: [RFC PATCH v1 5/8] vfio-ccw: Add support for the schib region

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

[Had two un-addressed QEMU comments on my todo list; let's do that now.]

On 11/20/19 6:13 AM, Cornelia Huck wrote:
> On Fri, 15 Nov 2019 04:34:34 +0100
> Eric Farman <farman@linux.ibm.com> wrote:
> 
>> From: Farhan Ali <alifm@linux.ibm.com>
>>
>> The schib region can be used to obtain the latest SCHIB from the host
>> passthrough subchannel. Since the guest SCHIB is virtualized,
>> we currently only update the path related information so that the
>> guest is aware of any path related changes when it issues the
>> 'stsch' instruction.
> 
> One important information here is that you tweak the generic stsch
> handling, although the behaviour remains the same for non-vfio
> subchannels.
> 
>>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>> ---
>>
>> Notes:
>>     v0->v1: [EF]
>>      - Change various incarnations of "update chp status" to
>>        "handle_store", to reflect the STSCH instruction that will
>>        drive this code
>>      - Remove temporary variable for casting/testing purposes in
>>        s390_ccw_store(), and add a block comment of WHY its there.
>>      - Add a few comments to vfio_ccw_handle_store()
>>
>>  hw/s390x/css.c              |  8 ++++--
>>  hw/s390x/s390-ccw.c         | 20 +++++++++++++
>>  hw/vfio/ccw.c               | 57 +++++++++++++++++++++++++++++++++++++
>>  include/hw/s390x/css.h      |  3 +-
>>  include/hw/s390x/s390-ccw.h |  1 +
>>  target/s390x/ioinst.c       |  3 +-
>>  6 files changed, 87 insertions(+), 5 deletions(-)
>>
>> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
>> index 844caab408..6ee6a96d75 100644
>> --- a/hw/s390x/css.c
>> +++ b/hw/s390x/css.c
>> @@ -1335,11 +1335,15 @@ static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)
>>      }
>>  }
>>  
>> -int css_do_stsch(SubchDev *sch, SCHIB *schib)
>> +IOInstEnding css_do_stsch(SubchDev *sch, SCHIB *schib)
>>  {
>> +    int ret = IOINST_CC_EXPECTED;
> 
> It's a bit useless initializing ret here, given that you use it right
> below :)
> 
>> +
> 
> Maybe add a comment here:
> 
>        /*
>         * For some subchannels, we may want to update parts of
>         * the schib (e.g. update path masks from the host device
>         * for passthrough subchannels).
>         */
> 
>> +    ret = s390_ccw_store(sch);
>> +
>>      /* Use current status. */
>>      copy_schib_to_guest(schib, &sch->curr_status);
>> -    return 0;
>> +    return ret;
>>  }
>>  
>>  static void copy_pmcw_from_guest(PMCW *dest, const PMCW *src)
>> diff --git a/hw/s390x/s390-ccw.c b/hw/s390x/s390-ccw.c
>> index 0c5a5b60bd..be0b379338 100644
>> --- a/hw/s390x/s390-ccw.c
>> +++ b/hw/s390x/s390-ccw.c
>> @@ -51,6 +51,26 @@ int s390_ccw_clear(SubchDev *sch)
>>      return cdc->handle_clear(sch);
>>  }
>>  
>> +IOInstEnding s390_ccw_store(SubchDev *sch)
>> +{
>> +    S390CCWDeviceClass *cdc = NULL;
>> +    int ret = IOINST_CC_EXPECTED;
>> +
>> +    /*
>> +     * This only applies to passthrough devices, so we can't unconditionally
>> +     * set this variable like we would for halt/clear.
> 
> Hm, can we maybe handle this differently? We have a generic ccw_cb in
> the subchannel structure for ccw interpretation; would it make sense to
> add a generic callback for stsch there as well?

Might make things a little cleaner, but I am not sure.  I'm going to
leave a todo here while we sort out the rest of the kernel code, so I
can try to put something more beneficial together.

> 
> (This works fine, though. Might want to add the check for halt/clear as
> well, but that might be a bit overkill.)
> 
>> +     */
>> +    if (object_dynamic_cast(OBJECT(sch->driver_data), TYPE_S390_CCW)) {
>> +        cdc = S390_CCW_DEVICE_GET_CLASS(sch->driver_data);
>> +    }
>> +
>> +    if (cdc && cdc->handle_store) {
>> +        ret = cdc->handle_store(sch);
>> +    }
>> +
>> +    return ret;
>> +}
>> +
>>  static void s390_ccw_get_dev_info(S390CCWDevice *cdev,
>>                                    char *sysfsdev,
>>                                    Error **errp)
>> diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
>> index 3e32bc1819..2ff223470f 100644
>> --- a/hw/vfio/ccw.c
>> +++ b/hw/vfio/ccw.c
>> @@ -41,6 +41,9 @@ struct VFIOCCWDevice {
>>      uint64_t async_cmd_region_size;
>>      uint64_t async_cmd_region_offset;
>>      struct ccw_cmd_region *async_cmd_region;
>> +    uint64_t schib_region_size;
>> +    uint64_t schib_region_offset;
>> +    struct ccw_schib_region *schib_region;
>>      EventNotifier io_notifier;
>>      bool force_orb_pfch;
>>      bool warned_orb_pfch;
>> @@ -124,6 +127,45 @@ again:
>>      }
>>  }
>>  
>> +static IOInstEnding vfio_ccw_handle_store(SubchDev *sch)
>> +{
>> +    S390CCWDevice *cdev = sch->driver_data;
>> +    VFIOCCWDevice *vcdev = DO_UPCAST(VFIOCCWDevice, cdev, cdev);
>> +    SCHIB *schib = &sch->curr_status;
>> +    struct ccw_schib_region *region = vcdev->schib_region;
>> +    SCHIB *s = (SCHIB *)region->schib_area;
> 
> You probably don't want to access region until after you checked it's
> not NULL :)

:)

> 
>> +    int ret;
>> +
>> +    /* schib region not available so nothing else to do */
>> +    if (!region) {
>> +        return IOINST_CC_EXPECTED;
>> +    }
>> +
>> +    memset(region, 0, sizeof(*region));
>> +    ret = pread(vcdev->vdev.fd, region, vcdev->schib_region_size,
>> +                vcdev->schib_region_offset);
>> +
>> +    if (ret == -1) {
>> +        /* Assume device is damaged, regardless of errno */
>> +        return IOINST_CC_NOT_OPERATIONAL;
> 
> Not sure that's correct; cc 3 for stsch indicates that there are no
> more subchannels with higher ids?

POPS says cc=3 on stsch if the subchannel is not operational, and it's
not operational if it's not provided to the channel subsystem.  So,
basically yes.  I guess his idea was to call out a problem on the kvm
side, so should we just return CC_EXPECTED here, as if there's no region?

> 
>> +    }
>> +
>> +    /*
>> +     * Selectively copy path-related bits of the SCHIB,
>> +     * rather than copying the entire struct.
>> +     */
>> +    schib->pmcw.pnom = s->pmcw.pnom;
>> +    schib->pmcw.lpum = s->pmcw.lpum;
>> +    schib->pmcw.pam = s->pmcw.pam;
>> +    schib->pmcw.pom = s->pmcw.pom;
>> +
>> +    if (s->scsw.flags & SCSW_FLAGS_MASK_PNO) {
>> +        schib->scsw.flags |= SCSW_FLAGS_MASK_PNO;
>> +    }
>> +
>> +    return IOINST_CC_EXPECTED;
>> +}
>> +
> 
> The rest of the patch looks good to me.
>