Re: [PATCH v26 05/17] vfio: Add VM state change handler to know state of VM

From: Cornelia Huck <cohuck@redhat.com>
To: Kirti Wankhede <kwankhede@nvidia.com>
Cc: cjia@nvidia.com, aik@ozlabs.ru, Zhengxiao.zx@alibaba-inc.com,
	shuangtai.tst@alibaba-inc.com, qemu-devel@nongnu.org,
	peterx@redhat.com, eauger@redhat.com, yi.l.liu@intel.com,
	quintela@redhat.com, ziye.yang@intel.com, armbru@redhat.com,
	mlevitsk@redhat.com, pasic@linux.ibm.com, felipe@nutanix.com,
	zhi.a.wang@intel.com, kevin.tian@intel.com, yan.y.zhao@intel.com,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	alex.williamson@redhat.com, changpeng.liu@intel.com,
	eskultet@redhat.com, Ken.Xue@amd.com,
	jonathan.davies@nutanix.com, pbonzini@redhat.com
Subject: Re: [PATCH v26 05/17] vfio: Add VM state change handler to know state of VM
Date: Tue, 20 Oct 2020 12:51:05 +0200	[thread overview]
Message-ID: <20201020125105.5cd790df.cohuck@redhat.com> (raw)
In-Reply-To: <3dd3fe95-c81a-de40-47b0-24f0772974d4@nvidia.com>

On Sun, 18 Oct 2020 01:54:56 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 9/29/2020 4:33 PM, Dr. David Alan Gilbert wrote:
> > * Cornelia Huck (cohuck@redhat.com) wrote:  
> >> On Wed, 23 Sep 2020 04:54:07 +0530
> >> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >>  
> >>> VM state change handler gets called on change in VM's state. This is used to set
> >>> VFIO device state to _RUNNING.
> >>>
> >>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >>> Reviewed-by: Neo Jia <cjia@nvidia.com>
> >>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >>> ---
> >>>   hw/vfio/migration.c           | 136 ++++++++++++++++++++++++++++++++++++++++++
> >>>   hw/vfio/trace-events          |   3 +-
> >>>   include/hw/vfio/vfio-common.h |   4 ++
> >>>   3 files changed, 142 insertions(+), 1 deletion(-)
> >>>  
> >>
> >> (...)
> >>  
> >>> +static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t mask,
> >>> +                                    uint32_t value)  
> >>
> >> I think I've mentioned that before, but this function could really
> >> benefit from a comment what mask and value mean.
> >>  
> 
> Adding a comment as:
> 
> /*
>   *  Write device_state field to inform the vendor driver about the 
> device state
>   *  to be transitioned to.
>   *  vbasedev: VFIO device
>   *  mask : bits set in the mask are preserved in device_state
>   *  value: bits set in the value are set in device_state
>   *  Remaining bits in device_state are cleared.
>   */

Maybe:

"Change the device_state register for device @vbasedev. Bits set in
@mask are preserved, bits set in @value are set, and bits not set in
either @mask or @value are cleared in device_state. If the register
cannot be accessed, the resulting state would be invalid, or the device
enters an error state, an error is returned." ?

> 
> 
> >>> +{
> >>> +    VFIOMigration *migration = vbasedev->migration;
> >>> +    VFIORegion *region = &migration->region;
> >>> +    off_t dev_state_off = region->fd_offset +
> >>> +                      offsetof(struct vfio_device_migration_info, device_state);
> >>> +    uint32_t device_state;
> >>> +    int ret;
> >>> +
> >>> +    ret = vfio_mig_read(vbasedev, &device_state, sizeof(device_state),
> >>> +                        dev_state_off);
> >>> +    if (ret < 0) {
> >>> +        return ret;
> >>> +    }
> >>> +
> >>> +    device_state = (device_state & mask) | value;
> >>> +
> >>> +    if (!VFIO_DEVICE_STATE_VALID(device_state)) {
> >>> +        return -EINVAL;
> >>> +    }
> >>> +
> >>> +    ret = vfio_mig_write(vbasedev, &device_state, sizeof(device_state),
> >>> +                         dev_state_off);
> >>> +    if (ret < 0) {
> >>> +        ret = vfio_mig_read(vbasedev, &device_state, sizeof(device_state),
> >>> +                          dev_state_off);
> >>> +        if (ret < 0) {
> >>> +            return ret;
> >>> +        }
> >>> +
> >>> +        if (VFIO_DEVICE_STATE_IS_ERROR(device_state)) {
> >>> +            hw_error("%s: Device is in error state 0x%x",
> >>> +                     vbasedev->name, device_state);
> >>> +            return -EFAULT;  
> >>
> >> Is -EFAULT a good return value here? Maybe -EIO?
> >>  
> 
> Ok. Changing to -EIO.
> 
> >>> +        }
> >>> +    }
> >>> +
> >>> +    vbasedev->device_state = device_state;
> >>> +    trace_vfio_migration_set_state(vbasedev->name, device_state);
> >>> +    return 0;
> >>> +}
> >>> +
> >>> +static void vfio_vmstate_change(void *opaque, int running, RunState state)
> >>> +{
> >>> +    VFIODevice *vbasedev = opaque;
> >>> +
> >>> +    if ((vbasedev->vm_running != running)) {
> >>> +        int ret;
> >>> +        uint32_t value = 0, mask = 0;
> >>> +
> >>> +        if (running) {
> >>> +            value = VFIO_DEVICE_STATE_RUNNING;
> >>> +            if (vbasedev->device_state & VFIO_DEVICE_STATE_RESUMING) {
> >>> +                mask = ~VFIO_DEVICE_STATE_RESUMING;  
> >>
> >> I've been staring at this for some time and I think that the desired
> >> result is
> >> - set _RUNNING
> >> - if _RESUMING was set, clear it, but leave the other bits intact  
> 
> Upto here, you're correct.
> 
> >> - if _RESUMING was not set, clear everything previously set
> >> This would really benefit from a comment (or am I the only one
> >> struggling here?)
> >>  
> 
> Here mask should be ~0. Correcting it.

Hm, now I'm confused. With value == _RUNNING, ~_RUNNING and ~0 as mask
should be equivalent, shouldn't they?

> 
> 
> >>> +            }
> >>> +        } else {
> >>> +            mask = ~VFIO_DEVICE_STATE_RUNNING;
> >>> +        }
> >>> +
> >>> +        ret = vfio_migration_set_state(vbasedev, mask, value);
> >>> +        if (ret) {
> >>> +            /*
> >>> +             * vm_state_notify() doesn't support reporting failure. If such
> >>> +             * error reporting support added in furure, migration should be
> >>> +             * aborted.  
> >>
> >>
> >> "We should abort the migration in this case, but vm_state_notify()
> >> currently does not support reporting failures."
> >>
> >> ?
> >>  
> 
> Ok. Updating comment as suggested here.
> 
> >> Can/should we mark the failing device in some way?  
> > 
> > I think you can call qemu_file_set_error on the migration stream to
> > force an error.
> >   
> 
> It should be as below, right?
> qemu_file_set_error(migrate_get_current()->to_dst_file, ret);

Does this indicate in any way which device was causing problems? (I'm
not sure how visible the error_report would be?)

> 
> 
> Thanks,
> Kirti
> 
> > Dave
> >   
> >>> +             */
> >>> +            error_report("%s: Failed to set device state 0x%x",
> >>> +                         vbasedev->name, value & mask);
> >>> +        }
> >>> +        vbasedev->vm_running = running;
> >>> +        trace_vfio_vmstate_change(vbasedev->name, running, RunState_str(state),
> >>> +                                  value & mask);
> >>> +    }
> >>> +}
> >>> +
> >>>   static int vfio_migration_init(VFIODevice *vbasedev,
> >>>                                  struct vfio_region_info *info)
> >>>   {  
> >>
> >> (...)  
> 