Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity

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

On Wed, 22 Sep 2021 13:38:51 +0300
Leon Romanovsky <leon@kernel.org> wrote:

> From: Yishai Hadas <yishaih@nvidia.com>
> 
> Add an API in the core layer to check migration state transition validity
> as part of a migration flow.
> 
> The valid transitions follow the expected usage as described in
> uapi/vfio.h and triggered by QEMU.
> 
> This ensures that all migration implementations follow a consistent
> migration state machine.
> 
> Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>  drivers/vfio/vfio.c  | 41 +++++++++++++++++++++++++++++++++++++++++
>  include/linux/vfio.h |  1 +
>  2 files changed, 42 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 3c034fe14ccb..c3ca33e513c8 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1664,6 +1664,47 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
>  	return 0;
>  }
>  
> +/**
> + * vfio_change_migration_state_allowed - Checks whether a migration state
> + *   transition is valid.
> + * @new_state: The new state to move to.
> + * @old_state: The old state.
> + * Return: true if the transition is valid.
> + */
> +bool vfio_change_migration_state_allowed(u32 new_state, u32 old_state)
> +{
> +	enum { MAX_STATE = VFIO_DEVICE_STATE_RESUMING };
> +	static const u8 vfio_from_state_table[MAX_STATE + 1][MAX_STATE + 1] = {
> +		[VFIO_DEVICE_STATE_STOP] = {
> +			[VFIO_DEVICE_STATE_RUNNING] = 1,
> +			[VFIO_DEVICE_STATE_RESUMING] = 1,
> +		},

Our state transition diagram is pretty weak on reachable transitions
out of the _STOP state, why do we select only these two as valid?

Consistent behavior to userspace is of course nice, but I wonder if we
were expecting a device reset to get us back to _RUNNING, or if the
drivers would make use of the protocol through which a driver can nak
(write error, no state change) or fault (_ERROR device state) a state
change.

There does need to be a way to get back to _RUNNING to support a
migration failure without a reset, but would that be from _SAVING
or from _STOP and what's our rationale for the excluded states?

I'll see if I can dig through emails to find what was intended to be
reachable from _STOP.  Kirti or Connie, do you recall?

Also, I think the _ERROR state is implicitly handled correctly here,
its value is >MAX_STATE so we can't transition into or out of it, but a
comment to indicate that it's been considered for this would be nice.

> +		[VFIO_DEVICE_STATE_RUNNING] = {
> +			[VFIO_DEVICE_STATE_STOP] = 1,
> +			[VFIO_DEVICE_STATE_SAVING] = 1,
> +			[VFIO_DEVICE_STATE_SAVING | VFIO_DEVICE_STATE_RUNNING] = 1,
> +		},

Shameer's comment is correct here, _RESUMING is a valid next state
since the default state is _RUNNING.

> +		[VFIO_DEVICE_STATE_SAVING] = {
> +			[VFIO_DEVICE_STATE_STOP] = 1,
> +			[VFIO_DEVICE_STATE_RUNNING] = 1,
> +		},

What's the rationale that we can't return to _SAVING|_RUNNING here?

> +		[VFIO_DEVICE_STATE_SAVING | VFIO_DEVICE_STATE_RUNNING] = {
> +			[VFIO_DEVICE_STATE_RUNNING] = 1,
> +			[VFIO_DEVICE_STATE_SAVING] = 1,
> +		},

Can't we always _STOP the device at any point?

> +		[VFIO_DEVICE_STATE_RESUMING] = {
> +			[VFIO_DEVICE_STATE_RUNNING] = 1,
> +			[VFIO_DEVICE_STATE_STOP] = 1,
> +		},

Couldn't it be possible to switch immediately to _RUNNING|_SAVING for
tracing purposes?  Or _SAVING, perhaps to validate the restored state
without starting the device?  Thanks,

Alex

> +	};
> +
> +	if (new_state > MAX_STATE || old_state > MAX_STATE)
> +		return false;
> +
> +	return vfio_from_state_table[old_state][new_state];
> +}
> +EXPORT_SYMBOL_GPL(vfio_change_migration_state_allowed);
> +
>  static long vfio_device_fops_unl_ioctl(struct file *filep,
>  				       unsigned int cmd, unsigned long arg)
>  {
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index b53a9557884a..e65137a708f1 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -83,6 +83,7 @@ extern struct vfio_device *vfio_device_get_from_dev(struct device *dev);
>  extern void vfio_device_put(struct vfio_device *device);
>  
>  int vfio_assign_device_set(struct vfio_device *device, void *set_id);
> +bool vfio_change_migration_state_allowed(u32 new_state, u32 old_state);
>  
>  /* events for the backend driver notify callback */
>  enum vfio_iommu_notify_type {