On Mon, 20 May 2019 12:29:56 -0400
Eric Farman <far...@linux.ibm.com> wrote:
> On 5/7/19 11:47 AM, 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 <coh...@redhat.com>
> > ---
> > hw/s390x/css.c | 27 +++++++--
> > hw/vfio/ccw.c | 110 +++++++++++++++++++++++++++++++++++-
> > include/hw/s390x/s390-ccw.h | 3 +
> > 3 files changed, 134 insertions(+), 6 deletions(-)
(...)
> > +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;
>
> Considering the serialization you added on the kernel side, what happens
> if another vcpu runs this code (or _halt) and clears the async region
> before the kernel code gains control from the pwrite() call below?
> Asked another way, there's nothing preventing us from issuing more than
> one asynchronous command concurrently, so how do we make sure the
> command gets to the kernel rather than "current command wins" ?
Hm... good question. But unfortunately not one I can answer quickly, so
I'll put off queuing this patch and just send a pull request without
it. It's not like we're in a hurry :)
>
> That possibly worrisome question aside, this seems generally fine.
Thanks for looking!
> > +
> > +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) {
> > + goto again;
> > + }
> > + error_report("vfio-ccw: write cmd region failed with errno=%d",
> > errno);
> > + ret = -errno;
> > + } else {
> > + ret = region->ret_code;
> > + }
> > + switch (ret) {
> > + case 0:
> > + case -ENODEV:
> > + case -EACCES:
> > + return 0;
> > + case -EFAULT:
> > + default:
> > + sch_gen_unit_exception(sch);
> > + css_inject_io_interrupt(sch);
> > + return 0;
> > + }
