Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity
From: Max Gurtovoy
Date: Wed Sep 29 2021 - 11:29:05 EST
On 9/29/2021 6:17 PM, Alex Williamson wrote:
On Wed, 29 Sep 2021 17:36:59 +0300
Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
On 9/29/2021 4:50 PM, Alex Williamson wrote:
On Wed, 29 Sep 2021 16:26:55 +0300
Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
On 9/29/2021 3:35 PM, Alex Williamson wrote:
On Wed, 29 Sep 2021 13:44:10 +0300
Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
On 9/28/2021 2:12 AM, Jason Gunthorpe wrote:
On Mon, Sep 27, 2021 at 04:46:48PM -0600, Alex Williamson wrote:
+ 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?
I have no particular opinion on specific states here, however adding
more states means more stuff for drivers to implement and more risk
driver writers will mess up this uAPI.
_STOP == 000b => Device Stopped, not saving or resuming (from UAPI).
This is the default initial state and not RUNNING.
The user application should move device from STOP => RUNNING or STOP =>
RESUMING.
Maybe we need to extend the comment in the UAPI file.
include/uapi/linux/vfio.h:
...
* +------- _RESUMING
* |+------ _SAVING
* ||+----- _RUNNING
* |||
* 000b => Device Stopped, not saving or resuming
* 001b => Device running, which is the default state
^^^^^^^^^^^^^^^^^^^^^^^^^^
...
* State transitions:
*
* _RESUMING _RUNNING Pre-copy Stop-and-copy _STOP
* (100b) (001b) (011b) (010b) (000b)
* 0. Running or default state
* |
^^^^^^^^^^^^^
...
* 0. Default state of VFIO device is _RUNNING when the user application starts.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The uAPI is pretty clear here. A default state of _STOP is not
compatible with existing devices and userspace that does not support
migration. Thanks,
Why do you need this state machine for userspace that doesn't support
migration ?
For userspace that doesn't support migration, there's one state,
_RUNNING. That's what we're trying to be compatible and consistent
with. Migration is an extension, not a base requirement.
Userspace without migration doesn't care about this state.
We left with kernel now. vfio-pci today doesn't support migration, right
? state is in theory is 0 (STOP).
This state machine is controlled by the migration SW. The drivers don't
move state implicitly.
mlx5-vfio-pci support migration and will work fine with non-migration SW
(it will stay with state = 0 unless someone will move it. but nobody
will) exactly like vfio-pci does today.
So where is the problem ?
So you have a device that's actively modifying its internal state,
performing I/O, including DMA (thereby dirtying VM memory), all while
in the _STOP state? And you don't see this as a problem?
I don't see how is it different from vfio-pci situation.
And you said you're worried from compatibility. I can't see a
compatibility issue here.
Maybe we need to rename STOP state. We can call it READY or LIVE or
NON_MIGRATION_STATE.
There's a major inconsistency if the migration interface is telling us
something different than we can actually observe through the behavior of
the device. Thanks,
Alex