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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




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




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux