Re: [Qemu-devel] [PATCH 2/2] vfio-ccw: support async command subregion

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

On 26/11/2018 10:58, Cornelia Huck wrote:
> On Fri, 23 Nov 2018 15:13:12 +0100
> Pierre Morel <pmorel@linux.ibm.com> wrote:
> 
>> On 22/11/2018 17:54, Cornelia Huck wrote:
>>> A vfio-ccw device may provide an async command subregion for
>>> issuing halt/clear subchannel requests. If it is present, use
>>> it for sending halt/clear request to the device; if not, fall
>>> back to emulation (as done today).
>>>
>>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>> ---
>>>    hw/s390x/css.c              |  27 +++++++--
>>>    hw/vfio/ccw.c               | 109 +++++++++++++++++++++++++++++++++++-
>>>    include/hw/s390x/s390-ccw.h |   3 +
>>>    3 files changed, 133 insertions(+), 6 deletions(-)
>>>
> 
>>> @@ -114,6 +120,87 @@ again:
>>>        }
>>>    }
>>>    
>>> +int vfio_ccw_handle_clear(SubchDev *sch)
>>> +{
>>> +    S390CCWDevice *cdev = sch->driver_data;
>>> +    VFIOCCWDevice *vcdev = DO_UPCAST(VFIOCCWDevice, cdev, cdev);
>>> +    struct ccw_cmd_region *region = vcdev->async_cmd_region;
>>> +    int ret;
>>> +
>>> +    if (!vcdev->async_cmd_region) {
>>> +        /* Async command region not available, fall back to emulation */
>>> +        return -ENOSYS;
>>> +    }
>>> +
>>> +    memset(region, 0, sizeof(*region));
>>> +    region->command = VFIO_CCW_ASYNC_CMD_CSCH;
>>> +
>>> +again:
>>> +    ret = pwrite(vcdev->vdev.fd, region,
>>> +                 vcdev->async_cmd_region_size, vcdev->async_cmd_region_offset);
>>> +    if (ret != vcdev->async_cmd_region_size) {
>>> +        if (errno == EAGAIN) {
>>
>>
>> Where do the EAGAIN come from?
> 
> It might be set by pwrite.

I saw that the man indicate this, and so we are legitimate to handle the 
fail case, but I did not find EAGAIN in the path of the write for 
accessing devices and I did not find it in the access to the CSS.

If we do not set it explicitly from the driver, the concern I have is: 
isn't it dangerous to try again and shouldn't we better abort?


> 
>>
>>
>>> +            goto again;
>>> +        }
>>> +        error_report("vfio-ccw: wirte I/O region failed with errno=%d", errno);
> 
> I should also fix up this message here and for hsch as well :)
> 
>>> +        ret = -errno;
>>> +    } else {
>>> +        ret = region->ret_code;
>>> +    }
>>> +    switch (ret) {
>>> +    case 0:
>>> +    case -ENODEV:
>>> +    case -EACCES:
>>
>> should never happen?
> 
> It should not happen; but it can nevertheless be returned, so...

I understand: nothing to clear :)


> 
>>
>>> +        return 0;
>>> +    case -EFAULT:
>>> +    default:
>>> +        sch_gen_unit_exception(sch);
>>> +        css_inject_io_interrupt(sch);
>>> +        return 0;
>>> +    }
>>> +}
>>> +
>>
>> otherwise LGTM
> 
> Thanks!
> 






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