Re: [PATCH v6 09/10] hisi_acc_vfio_pci: Add support for VFIO live migration

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 28 Feb 2022 14:05:20 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Feb 28, 2022 at 06:01:44PM +0000, Shameerali Kolothum Thodi wrote:
> 
> > +static long hisi_acc_vf_save_unl_ioctl(struct file *filp,
> > +                                      unsigned int cmd, unsigned long arg)
> > +{
> > +       struct hisi_acc_vf_migration_file *migf = filp->private_data;
> > +       loff_t *pos = &filp->f_pos;
> > +       struct vfio_device_mig_precopy precopy;
> > +       unsigned long minsz;
> > +
> > +       if (cmd != VFIO_DEVICE_MIG_PRECOPY)
> > +               return -EINVAL;  
> 
> ENOTTY
> 
> > +
> > +       minsz = offsetofend(struct vfio_device_mig_precopy, dirty_bytes);
> > +
> > +       if (copy_from_user(&precopy, (void __user *)arg, minsz))
> > +               return -EFAULT;
> > +       if (precopy.argsz < minsz)
> > +               return -EINVAL;
> > +
> > +       mutex_lock(&migf->lock);
> > +       if (*pos > migf->total_length) {
> > +               mutex_unlock(&migf->lock);
> > +               return -EINVAL;
> > +       }
> > +
> > +       precopy.dirty_bytes = 0;
> > +       precopy.initial_bytes = migf->total_length - *pos;
> > +       mutex_unlock(&migf->lock);
> > +       return copy_to_user((void __user *)arg, &precopy, minsz) ? -EFAULT : 0;
> > +}  
> 
> Yes
> 
> And I noticed this didn't include the ENOMSG handling, read() should
> return ENOMSG when it reaches EOS for the pre-copy:
> 
> + * During pre-copy the migration data FD has a temporary "end of stream" that is
> + * reached when both initial_bytes and dirty_byte are zero. For instance, this
> + * may indicate that the device is idle and not currently dirtying any internal
> + * state. When read() is done on this temporary end of stream the kernel driver
> + * should return ENOMSG from read(). Userspace can wait for more data (which may
> + * never come) by using poll.

I'm confused by your previous reply that the use of curr_state should
be eliminated, isn't this ioctl only valid while the device is in the
PRE_COPY or PRE_COPY_P2P states?  Otherwise the STOP_COPY state would
have some expectation to be able to use this ioctl for devices
supporting PRE_COPY.  I'd like to see the uapi clarify exactly what
states allow this ioctl and define the behavior of the ioctl when
transitioning out of those states with an open data_fd, ie. is it
defined to return an -errno once in STOP_COPY?  Thanks,

Alex