Re: [qemu-s390x] [PATCH 2/2] vfio-ccw: support async command subregion

[Pierre Morel]

On 26/11/2018 10:58, Cornelia Huck wrote:
> On Fri, 23 Nov 2018 15:13:12 +0100
> Pierre Morel <address@hidden> 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 <address@hidden>
> > > ---
> > >    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