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Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device


From: Kirti Wankhede
Subject: Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device state
Date: Tue, 17 Dec 2019 11:58:44 +0530
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2



On 12/17/2019 4:14 AM, Alex Williamson wrote:
On Tue, 17 Dec 2019 01:51:36 +0530
Kirti Wankhede <address@hidden> wrote:

- Defined MIGRATION region type and sub-type.

- Defined vfio_device_migration_info structure which will be placed at 0th
   offset of migration region to get/set VFIO device related information.
   Defined members of structure and usage on read/write access.

- Defined device states and added state transition details in the comment.

- Added sequence to be followed while saving and resuming VFIO device state

Signed-off-by: Kirti Wankhede <address@hidden>
Reviewed-by: Neo Jia <address@hidden>
---
  include/uapi/linux/vfio.h | 180 ++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 180 insertions(+)

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 9e843a147ead..a0817ba267c1 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -305,6 +305,7 @@ struct vfio_region_info_cap_type {
  #define VFIO_REGION_TYPE_PCI_VENDOR_MASK      (0xffff)
  #define VFIO_REGION_TYPE_GFX                    (1)
  #define VFIO_REGION_TYPE_CCW                  (2)
+#define VFIO_REGION_TYPE_MIGRATION              (3)
/* sub-types for VFIO_REGION_TYPE_PCI_* */ @@ -379,6 +380,185 @@ struct vfio_region_gfx_edid {
  /* sub-types for VFIO_REGION_TYPE_CCW */
  #define VFIO_REGION_SUBTYPE_CCW_ASYNC_CMD     (1)
+/* sub-types for VFIO_REGION_TYPE_MIGRATION */
+#define VFIO_REGION_SUBTYPE_MIGRATION           (1)
+
+/*
+ * Structure vfio_device_migration_info is placed at 0th offset of
+ * VFIO_REGION_SUBTYPE_MIGRATION region to get/set VFIO device related 
migration
+ * information. Field accesses from this structure are only supported at their
+ * native width and alignment, otherwise the result is undefined and vendor
+ * drivers should return an error.
+ *
+ * device_state: (read/write)
+ *      To indicate vendor driver the state VFIO device should be transitioned
+ *      to. If device state transition fails, write on this field return error.
+ *      It consists of 3 bits:
+ *      - If bit 0 set, indicates _RUNNING state. When its clear, that 
indicates

s/its/it's/

+ *        _STOP state. When device is changed to _STOP, driver should stop
+ *        device before write() returns.
+ *      - If bit 1 set, indicates _SAVING state. When set, that indicates 
driver
+ *        should start gathering device state information which will be 
provided
+ *        to VFIO user space application to save device's state.
+ *      - If bit 2 set, indicates _RESUMING state. When set, that indicates
+ *        prepare to resume device, data provided through migration region
+ *        should be used to resume device.
+ *      Bits 3 - 31 are reserved for future use. User should perform
+ *      read-modify-write operation on this field.
+ *
+ *  +------- _RESUMING
+ *  |+------ _SAVING
+ *  ||+----- _RUNNING
+ *  |||
+ *  000b => Device Stopped, not saving or resuming
+ *  001b => Device running state, default state
+ *  010b => Stop Device & save device state, stop-and-copy state
+ *  011b => Device running and save device state, pre-copy state
+ *  100b => Device stopped and device state is resuming
+ *  101b => Invalid state

Eventually this would be intended for post-copy, if supported by the
device, right?


No, as per Yan mentioned in earlier version, _RESUMING + _RUNNING can't be used for post-copy. New flag will be required for post-copy.

https://www.mail-archive.com/address@hidden/msg658768.html

+ *  110b => Invalid state
+ *  111b => Invalid state
+ *
+ * State transitions:
+ *
+ *              _RESUMING  _RUNNING    Pre-copy    Stop-and-copy   _STOP
+ *                (100b)     (001b)     (011b)        (010b)       (000b)
+ * 0. Running or Default state
+ *                             |
+ *
+ * 1. Normal Shutdown

Optional, userspace is under no obligation.

+ *                             |------------------------------------->|
+ *
+ * 2. Save state or Suspend
+ *                             |------------------------->|---------->|
+ *
+ * 3. Save state during live migration
+ *                             |----------->|------------>|---------->|
+ *
+ * 4. Resuming
+ *                  |<---------|
+ *
+ * 5. Resumed
+ *                  |--------->|
+ *
+ * 0. Default state of VFIO device is _RUNNNG when VFIO application starts.
+ * 1. During normal VFIO application shutdown, vfio device state changes
+ *    from _RUNNING to _STOP.

We cannot impose this requirement on existing userspace.  Userspace may
perform this action, but they are not required to and the vendor driver
must not require it.

Updated comment.


+ * 2. When VFIO application save state or suspend application, VFIO device
+ *    state transition is from _RUNNING to stop-and-copy state and then to
+ *    _STOP.
+ *    On state transition from _RUNNING to stop-and-copy, driver must
+ *    stop device, save device state and send it to application through
+ *    migration region.
+ *    On _RUNNING to stop-and-copy state transition failure, application should
+ *    set VFIO device state to _RUNNING.

A state transition failure means that the user's write to device_state
failed, so is it the user's responsibility to set the next state?

Right.

 Why
is it necessarily _RUNNING vs _STOP?


While changing From pre-copy to stop-and-copy transition, device is still running, only saving of device state started. Now if transition to stop-and-copy fails, from user point of view application or VM is still running, device state should be set to _RUNNING so that whatever the application/VM is running should continue at source.


+ * 3. In VFIO application live migration, state transition is from _RUNNING
+ *    to pre-copy to stop-and-copy to _STOP.
+ *    On state transition from _RUNNING to pre-copy, driver should start
+ *    gathering device state while application is still running and send device
+ *    state data to application through migration region.
+ *    On state transition from pre-copy to stop-and-copy, driver must stop
+ *    device, save device state and send it to application through migration
+ *    region.
+ *    On any failure during any of these state transition, VFIO device state
+ *    should be set to _RUNNING.

Same comment as above regarding next state on failure.


If application or VM migration fails, it should continue to run at source. In case of VM, guest user isn't aware of migration, and from his point VM should be running.

Also, it seems like it's the vendor driver's discretion to actually
provide data during the pre-copy phase.  As we've defined it, the
vendor driver needs to participate in the migration region regardless,
they might just always report no pending_bytes until we enter
stop-and-copy.


Yes. And if pending_bytes are reported as 0 in pre-copy by vendor driver then QEMU doesn't reiterate for that device.

+ * 4. To start resuming phase, VFIO device state should be transitioned from
+ *    _RUNNING to _RESUMING state.
+ *    In _RESUMING state, driver should use received device state data through
+ *    migration region to resume device.
+ *    On failure during this state transition, application should set _RUNNING
+ *    state.

Same comment regarding setting next state after failure.

If device couldn't be transitioned to _RESUMING, then it should be set to default state, that is _RUNNING.


+ * 5. On providing saved device data to driver, appliation should change state
+ *    from _RESUMING to _RUNNING.
+ *    On failure to transition to _RUNNING state, VFIO application should reset
+ *    the device and set _RUNNING state so that device doesn't remain in 
unknown
+ *    or bad state. On reset, driver must reset device and device should be
+ *    available in default usable state.

Didn't we discuss that the reset ioctl should return the device to the
initial state, including the transition to _RUNNING?

Yes, that's default usable state, rewording it to initial state.

 Also, as above,
it's the user write that triggers the failure, this register is listed
as read-write, so what value does the vendor driver report for the
state when read after a transition failure?  Is it reported as _RESUMING
as it was prior to the attempted transition, or may the invalid states
be used by the vendor driver to indicate the device is broken?


If transition as failed, device should report its previous state and reset device should bring back to usable _RUNNING state.

+ *
+ * pending bytes: (read only)
+ *      Number of pending bytes yet to be migrated from vendor driver
+ *
+ * data_offset: (read only)
+ *      User application should read data_offset in migration region from where
+ *      user application should read device data during _SAVING state or write
+ *      device data during _RESUMING state. See below for detail of sequence to
+ *      be followed.
+ *
+ * data_size: (read/write)
+ *      User application should read data_size to get size of data copied in
+ *      bytes in migration region during _SAVING state and write size of data
+ *      copied in bytes in migration region during _RESUMING state.
+ *
+ * Migration region looks like:
+ *  ------------------------------------------------------------------
+ * |vfio_device_migration_info|    data section                      |
+ * |                          |     ///////////////////////////////  |
+ * ------------------------------------------------------------------
+ *   ^                              ^
+ *  offset 0-trapped part        data_offset
+ *
+ * Structure vfio_device_migration_info is always followed by data section in
+ * the region, so data_offset will always be non-0. Offset from where data is
+ * copied is decided by kernel driver, data section can be trapped or mapped
+ * or partitioned, depending on how kernel driver defines data section.
+ * Data section partition can be defined as mapped by sparse mmap capability.
+ * If mmapped, then data_offset should be page aligned, where as initial 
section
+ * which contain vfio_device_migration_info structure might not end at offset
+ * which is page aligned. The user is not required to access via mmap 
regardless
+ * of the region mmap capabilities.
+ * Vendor driver should decide whether to partition data section and how to
+ * partition the data section. Vendor driver should return data_offset
+ * accordingly.
+ *
+ * Sequence to be followed for _SAVING|_RUNNING device state or pre-copy phase
+ * and for _SAVING device state or stop-and-copy phase:
+ * a. read pending_bytes, indicates start of new iteration to get device data.
+ *    If there was previous iteration, then this read operation indicates
+ *    previous iteration is done. If pending_bytes > 0, go through below steps.
+ * b. read data_offset, indicates kernel driver to make data available through
+ *    data section. Kernel driver should return this read operation only after
+ *    data is available from (region + data_offset) to (region + data_offset +
+ *    data_size).
+ * c. read data_size, amount of data in bytes available through migration
+ *    region.
+ * d. read data of data_size bytes from (region + data_offset) from migration
+ *    region.
+ * e. process data.
+ * f. Loop through a to e.

It seems we always need to end an iteration by reading pending_bytes to
signal to the vendor driver to release resources, so should the end of
the loop be:

e. Read pending_bytes
f. Goto b. or optionally restart next iteration at a.

I think this is defined such that reading data_offset commits resources
and reading pending_bytes frees them, allowing userspace to restart at
reading pending_bytes with no side-effects.  Therefore reading
pending_bytes repeatedly is supported.  Is the same true for
data_offset and data_size?  It seems reasonable that the vendor driver
can simply return offset and size for the current buffer if the user
reads these more than once.


Right.


How is a protocol or device error signaled?  For example, we can have a
user error where they read data_size before data_offset.  Should the
vendor driver generate a fault reading data_size in this case.  We can
also have internal errors in the vendor driver, should the vendor
driver use a special errno or update device_state autonomously to
indicate such an error?

If there is any error during the sequence, vendor driver can return error code for next read/write operation, that will terminate the loop and migration would fail.


I believe it's also part of the intended protocol that the user can
transition from _SAVING|_RUNNING to _SAVING at any point, regardless of
pending_bytes.  This should be noted.


Ok. Updating comment.

+ *
+ * Sequence to be followed while _RESUMING device state:
+ * While data for this device is available, repeat below steps:
+ * a. read data_offset from where user application should write data.
+ * b. write data of data_size to migration region from data_offset.

Whose's data_size, the _SAVING end or the _RESUMING end?  I think this
is intended to be the transaction size from the _SAVING source,

Not necessarily. data_size could be MIN(transaction size of source, migration data section). If migration data section is smaller than data packet size at source, then it has to be broken and iteratively sent.

but it
could easily be misinterpreted as reading data_size on the _RESUMING
end.

+ * c. write data_size which indicates vendor driver that data is written in
+ *    staging buffer. Vendor driver should read this data from migration
+ *    region and resume device's state.

I think we also need to define the error protocol.  The user could
mis-order transactions or there could be an internal error in the
vendor driver or device.  Are all read(2)/write(2) operations
susceptible to defined errnos to signal this?

Yes.

 Is it reflected in
device_state?

No.

What's the recovery protocol?


On read()/write() failure user should take necessary action.


+ *
+ * For user application, data is opaque. User should write data in the same
+ * order as received.

Order and transaction size, ie. each data_size chunk is indivisible by
the user.

Transaction size can differ, but order should remain same.


+ */
+
+struct vfio_device_migration_info {
+       __u32 device_state;         /* VFIO device state */
+#define VFIO_DEVICE_STATE_STOP      (1 << 0)
+#define VFIO_DEVICE_STATE_RUNNING   (1 << 0)

Huh?  We should probably just refer to it consistently, ie. _RUNNING
and !_RUNNING, otherwise we have the incongruity that setting the _STOP
value is actually the opposite of the necessary logic value (_STOP = 1
is _RUNNING, _STOP = 0 is !_RUNNING).

Ops, my mistake, forgot to update to
#define VFIO_DEVICE_STATE_STOP      (0)


+#define VFIO_DEVICE_STATE_SAVING    (1 << 1)
+#define VFIO_DEVICE_STATE_RESUMING  (1 << 2)
+#define VFIO_DEVICE_STATE_MASK      (VFIO_DEVICE_STATE_RUNNING | \
+                                    VFIO_DEVICE_STATE_SAVING |  \
+                                    VFIO_DEVICE_STATE_RESUMING)
+
+#define VFIO_DEVICE_STATE_INVALID_CASE1    (VFIO_DEVICE_STATE_SAVING | \
+                                           VFIO_DEVICE_STATE_RESUMING)
+
+#define VFIO_DEVICE_STATE_INVALID_CASE2    (VFIO_DEVICE_STATE_RUNNING | \
+                                           VFIO_DEVICE_STATE_RESUMING)

Gack, we fixed these in the last iteration!


That solution doesn't scale when new flags will be added. I still prefer to define as above.

Thanks,
Kirti

+       __u32 reserved;
+       __u64 pending_bytes;
+       __u64 data_offset;
+       __u64 data_size;
+} __attribute__((packed));
+
  /*
   * The MSIX mappable capability informs that MSIX data of a BAR can be mmapped
   * which allows direct access to non-MSIX registers which happened to be 
within

Thanks,
Alex




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