Re: [PATCH v12 Kernel 1/7] vfio: KABI for migration interface for device state

Kirti Wankhede
2020-02-12T20:56:34Z

On 2/10/2020 10:55 PM, Alex Williamson wrote:
> On Sat, 8 Feb 2020 01:12:28 +0530
> Kirti Wankhede <kwankhede@nvidia.com> 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 state transition details.
>>
>> - Defined sequence to be followed while saving and resuming VFIO device.
>>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>> ---
>> include/uapi/linux/vfio.h | 208 ++++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 208 insertions(+)
>>
>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>> index 9e843a147ead..572242620ce9 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 +380,213 @@ 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)
>> + * - User application writes this field to inform vendor driver about the
>> + * device state to be transitioned to.
>> + * - Vendor driver should take necessary actions to change device state.
>> + * On successful transition to given state, vendor driver should return
>> + * success on write(device_state, state) system call. If device state
>> + * transition fails, vendor driver should return error, -EFAULT.
>
> s/error, -EFAULT/an appropriate -errno for the fault condition/
>
>> + * - On user application side, if device state transition fails, i.e. if
>> + * write(device_state, state) returns error, read device_state again to
>> + * determine the current state of the device from vendor driver.
>> + * - Vendor driver should return previous state of the device unless vendor
>> + * driver has encountered an internal error, in which case vendor driver
>> + * may report the device_state VFIO_DEVICE_STATE_ERROR.
>> + * - User application must use the device reset ioctl in order to recover
>> + * the device from VFIO_DEVICE_STATE_ERROR state. If the device is
>> + * indicated in a valid device state via reading device_state, the user
>> + * application may decide attempt to transition the device to any valid
>> + * state reachable from the current state or terminate itself.
>> + *
>> + * device_state consists of 3 bits:
>> + * - If bit 0 set, indicates _RUNNING state. When it's clear, that
>> + * indicates _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 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. In order to preserve them,
>> + * user application should perform read-modify-write operation on this
>> + * field when modifying the specified bits.
>> + *
>> + * +------- _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
>> + * 110b => Error 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)
>> + * |------------------------------------->|
>> + *
>> + * 2. Save state or Suspend
>> + * |------------------------->|---------->|
>> + *
>> + * 3. Save state during live migration
>> + * |----------->|------------>|---------->|
>> + *
>> + * 4. Resuming
>> + * |<---------|
>> + *
>> + * 5. Resumed
>> + * |--------->|
>> + *
>> + * 0. Default state of VFIO device is _RUNNNG when user application starts.
>> + * 1. During normal user application shutdown, vfio device state changes
>> + * from _RUNNING to _STOP. This is optional, user application may or may not
>> + * perform this state transition and vendor driver may not need.
>
> s/may not need/must not require, but must support this transition/
>
>> + * 2. When user application save state or suspend application, device state
>> + * transitions 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. Sequence to be followed for such transition is given
>> + * below.
>> + * 3. In user application live migration, state transitions 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 user application through
>> + * migration region.
>> + * Sequence to be followed for above two transitions is given below.
>
> Perhaps adding something like "Vendor drivers must support the pre-copy
> state even for implementations where no data is provided to the user
> until the stop-and-copy state. The user must not be required to
> consume all migration data prior to transitioning to a new state,
> including the stop-and-copy state."
>
>> + * 4. To start resuming phase, 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.
>> + * 5. On providing saved device data to driver, application should change state
>> + * from _RESUMING to _RUNNING.
>> + *
>> + * 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.
>> + * Repeatative read on pending_bytes at this stage should have no side
>> + * effect.
>
> s/Repeatative/Repeated/
>
>> + * If pending_bytes == 0, user application should not iterate to get data
>> + * for that device.
>> + * If pending_bytes > 0, go through below steps.
>> + * b. read data_offset, indicates vendor driver to make data available through
>> + * data section. Vendor 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.
>> + * Read on data_offset and data_size should return offset and size of current
>> + * buffer if user application reads those more than once here.
>> + * d. read data of data_size bytes from (region + data_offset) from migration
>> + * region.
>> + * e. process data.
>> + * f. read pending_bytes, this read operation indicates data from previous
>> + * iteration had read. If pending_bytes > 0, goto step b.
>> + *
>> + * If there is any error during the above sequence, vendor driver can return
>> + * error code for next read()/write() operation, that will terminate the loop
>> + * and user should take next necessary action, for example, fail migration or
>> + * terminate user application.
>> + *
>> + * User application can transition from _SAVING|_RUNNING (pre-copy state) to
>> + * _SAVING (stop-and-copy) state regardless of pending bytes.
>
> Ok, you cover one of my concerns above here. Maybe doesn't hurt to
> mention in both places.
>
>> + * User application should iterate in _SAVING (stop-and-copy) until
>> + * pending_bytes is 0.
>> + *
>> + * 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. Data size
>> + * should be data packet size at source during _SAVING.
>
> I find the reference to data_size a bit confusing in this wording,
> almost as if it's implied that the user reads data_size on the target.
> What if we changed it a little:
>
> b. write migration data starting at migration region + data_offset for
> length determined by data_size from the migration source.
>
>> + * c. write data_size which indicates vendor driver that data is written in
>> + * migration region. Vendor driver should read this data from migration
>> + * region and resume device's state.
>
> Perhaps "Vendor driver should apply the user provided migration region
> data towards the device resume state"?
>

Ok. Updating as per all above comments.

>> + *
>> + * For user application, data is opaque. User application should write data in
>> + * the same order as received and should of same transaction size at source.
>
> Great!
>
>> + */
>> +
>> +struct vfio_device_migration_info {
>> + __u32 device_state; /* VFIO device state */
>> +#define VFIO_DEVICE_STATE_STOP (0)
>> +#define VFIO_DEVICE_STATE_RUNNING (1 << 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_VALID(state) \
>> + (state & VFIO_DEVICE_STATE_RESUMING ? \
>> + (state & VFIO_DEVICE_STATE_MASK) == VFIO_DEVICE_STATE_RESUMING : 1)
>> +
>> +#define VFIO_DEVICE_STATE_ERROR \
>> + (VFIO_DEVICE_STATE_SAVING | VFIO_DEVICE_STATE_RESUMING)
>
> It looks like this isn't used in this series, so I'm not sure the
> intention of this macro, but I think we decided to only use 110b as the
> "error" state. So should this be something like
>
> #define VFIO_DEVICE_STATE_IS_ERROR(state) \
> (state & VFIO_DEVICE_STATE_MASK == (VFIO_DEVICE_STATE_SAVING | \
> VFIO_DEVICE_STATE_RESUMING))
>
> Or if this was intended to be used in setting the device_state to
> error, perhaps
>
> #define VFIO_DEVICE_STATE_SET_ERROR(state) \
> ((state & ~VFIO_DEVICE_STATE_MASK) | VFIO_DEVICE_SATE_SAVING | \
> VFIO_DEVICE_STATE_RESUMING)

This is also intended to to set device_state, vendor driver would set
error state. Adding both above macros.

>> +
>> + __u32 reserved;
>
> Can we specify this reserved field as reads return zero, writes are
> ignored so that we give ourselves the opportunity to re-purpose it
> later?
>
>

Ok. Adding

Thanks,
Kirti

> + __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
>