Re: [PATCH v14 Kernel 1/7] vfio: KABI for migration interface for device state

Alex Williamson
2020-03-19T13:09:37Z

On Thu, 19 Mar 2020 01:05:54 -0400
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Thu, Mar 19, 2020 at 11:49:26AM +0800, Alex Williamson wrote:
> > On Wed, 18 Mar 2020 21:17:03 -0400
> > Yan Zhao <yan.y.zhao@intel.com> wrote:
> >
> > > On Thu, Mar 19, 2020 at 03:41:08AM +0800, Kirti Wankhede wrote:
> > > > - Defined MIGRATION region type and sub-type.
> > > >
> > > > - Defined vfio_device_migration_info structure which will be placed at the
> > > > 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 | 227 ++++++++++++++++++++++++++++++++++++++++++++++
> > > > 1 file changed, 227 insertions(+)
> > > >
> > > > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > > > index 9e843a147ead..d0021467af53 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 +380,232 @@ 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)
> > > > +
> > > > +/*
> > > > + * The structure vfio_device_migration_info is placed at the 0th offset of
> > > > + * the VFIO_REGION_SUBTYPE_MIGRATION region to get and 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)
> > > > + * - The user application writes to this field to inform the vendor driver
> > > > + * about the device state to be transitioned to.
> > > > + * - The vendor driver should take the necessary actions to change the
> > > > + * device state. After successful transition to a given state, the
> > > > + * vendor driver should return success on write(device_state, state)
> > > > + * system call. If the device state transition fails, the vendor driver
> > > > + * should return an appropriate -errno for the fault condition.
> > > > + * - On the user application side, if the device state transition fails,
> > > > + * that is, if write(device_state, state) returns an error, read
> > > > + * device_state again to determine the current state of the device from
> > > > + * the vendor driver.
> > > > + * - The vendor driver should return previous state of the device unless
> > > > + * the vendor driver has encountered an internal error, in which case
> > > > + * the vendor driver may report the device_state VFIO_DEVICE_STATE_ERROR.
> > > > + * - The user application must use the device reset ioctl to recover the
> > > > + * device from VFIO_DEVICE_STATE_ERROR state. If the device is
> > > > + * indicated to be in a valid device state by reading device_state, the
> > > > + * user application may 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 is set, it indicates the _RUNNING state. If bit 0 is clear,
> > > > + * it indicates the _STOP state. When the device state is changed to
> > > > + * _STOP, driver should stop the device before write() returns.
> > > > + * - If bit 1 is set, it indicates the _SAVING state, which means that the
> > > > + * driver should start gathering device state information that will be
> > > > + * provided to the VFIO user application to save the device's state.
> > > > + * - If bit 2 is set, it indicates the _RESUMING state, which means that
> > > > + * the driver should prepare to resume the device. Data provided through
> > > > + * the migration region should be used to resume the device.
> > > > + * Bits 3 - 31 are reserved for future use. To preserve them, the user
> > > > + * application should perform a 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, which is the default state
> > > > + * 010b => Stop the device & save the device state, stop-and-copy state
> > > > + * 011b => Device running and save the device state, pre-copy state
> > > > + * 100b => Device stopped and the 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 the state or suspend
> > > > + * |------------------------->|---------->|
> > > > + *
> > > > + * 3. Save the state during live migration
> > > > + * |----------->|------------>|---------->|
> > > > + *
> > > > + * 4. Resuming
> > > > + * |<---------|
> > > > + *
> > > > + * 5. Resumed
> > > > + * |--------->|
> > > > + *
> > > > + * 0. Default state of VFIO device is _RUNNNG when the user application starts.
> > > > + * 1. During normal shutdown of the user application, the user application may
> > > > + * optionally change the VFIO device state from _RUNNING to _STOP. This
> > > > + * transition is optional. The vendor driver must support this transition but
> > > > + * must not require it.
> > > > + * 2. When the user application saves state or suspends the application, the
> > > > + * device state transitions from _RUNNING to stop-and-copy and then to _STOP.
> > > > + * On state transition from _RUNNING to stop-and-copy, driver must stop the
> > > > + * device, save the device state and send it to the application through the
> > > > + * migration region. The sequence to be followed for such transition is given
> > > > + * below.
> > > > + * 3. In live migration of user application, the state transitions from _RUNNING
> > > > + * to pre-copy, to stop-and-copy, and to _STOP.
> > > > + * On state transition from _RUNNING to pre-copy, the driver should start
> > > > + * gathering the device state while the application is still running and send
> > > > + * the device state data to application through the migration region.
> > > > + * On state transition from pre-copy to stop-and-copy, the driver must stop
> > > > + * the device, save the device state and send it to the user application
> > > > + * through the migration region.
> > > > + * Vendor drivers must support the pre-copy state even for implementations
> > > > + * where no data is provided to the user before the stop-and-copy state. The
> > > > + * user must not be required to consume all migration data before the device
> > > > + * transitions to a new state, including the stop-and-copy state.
> > > > + * The sequence to be followed for above two transitions is given below.
> > > > + * 4. To start the resuming phase, the device state should be transitioned from
> > > > + * the _RUNNING to the _RESUMING state.
> > > > + * In the _RESUMING state, the driver should use the device state data
> > > > + * received through the migration region to resume the device.
> > > > + * 5. After providing saved device data to the driver, the application should
> > > > + * change the state from _RESUMING to _RUNNING.
> > > > + *
> > > > + * reserved:
> > > > + * Reads on this field return zero and writes are ignored.
> > > > + *
> > > > + * pending_bytes: (read only)
> > > > + * The number of pending bytes still to be migrated from the vendor driver.
> > > > + *
> > > > + * data_offset: (read only)
> > > > + * The user application should read data_offset in the migration region
> > > > + * from where the user application should read the device data during the
> > > > + * _SAVING state or write the device data during the _RESUMING state. See
> > > > + * below for details of sequence to be followed.
> > > > + *
> > > > + * data_size: (read/write)
> > > > + * The user application should read data_size to get the size in bytes of
> > > > + * the data copied in the migration region during the _SAVING state and
> > > > + * write the size in bytes of the data copied in the migration region
> > > > + * during the _RESUMING state.
> > > > + *
> > > > + * The format of the migration region is as follows:
> > > > + * ------------------------------------------------------------------
> > > > + * |vfio_device_migration_info| data section |
> > > > + * | | /////////////////////////////// |
> > > > + * ------------------------------------------------------------------
> > > > + * ^ ^
> > > > + * offset 0-trapped part data_offset
> > > > + *
> > > > + * The structure vfio_device_migration_info is always followed by the data
> > > > + * section in the region, so data_offset will always be nonzero. The offset
> > > > + * from where the data is copied is decided by the kernel driver. The data
> > > > + * section can be trapped, mapped, or partitioned, depending on how the kernel
> > > > + * driver defines the data section. The data section partition can be defined
> > > > + * as mapped by the sparse mmap capability. If mmapped, data_offset should be
> > > > + * page aligned, whereas initial section which contains the
> > > > + * vfio_device_migration_info structure, might not end at the offset, which is
> > > > + * page aligned. The user is not required to access through mmap regardless
> > > > + * of the capabilities of the region mmap.
> > > > + * The vendor driver should determine whether and how to partition the data
> > > > + * section. The vendor driver should return data_offset accordingly.
> > > > + *
> > > > + * The sequence to be followed for the _SAVING|_RUNNING device state or
> > > > + * pre-copy phase and for the _SAVING device state or stop-and-copy phase is as
> > > > + * follows:
> > > > + * a. Read pending_bytes, indicating the start of a new iteration to get device
> > > > + * data. Repeated read on pending_bytes at this stage should have no side
> > > > + * effects.
> > > > + * If pending_bytes == 0, the user application should not iterate to get data
> > > > + * for that device.
> > > > + * If pending_bytes > 0, perform the following steps.
> > > > + * b. Read data_offset, indicating that the vendor driver should make data
> > > > + * available through the data section. The 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, which is the amount of data in bytes available through
> > > > + * the migration region.
> > > > + * Read on data_offset and data_size should return the offset and size of
> > > > + * the current buffer if the user application reads data_offset and
> > > > + * data_size more than once here.
> > > If data region is mmaped, merely reading data_offset and data_size
> > > cannot let kernel know what are correct values to return.
> > > Consider to add a read operation which is trapped into kernel to let
> > > kernel exactly know it needs to move to the next offset and update data_size
> > > ?
> >
> > Both operations b. and c. above are to trapped registers, operation d.
> > below may potentially be to an mmap'd area, which is why we have step
> > f. which indicates to the vendor driver that the data has been
> > consumed. Does that address your concern? Thanks,
> >
> No. :)
> the problem is about semantics of data_offset, data_size, and
> pending_bytes.
> b and c do not tell kernel that the data is read by user.
> so, without knowing step d happen, kernel cannot update pending_bytes to
> be returned in step f.

Sorry, I'm still not understanding, I see step f. as the indicator
you're looking for. The user reads pending_bytes to indicate the data
in the migration area has been consumed. The vendor driver updates its
internal state on that read and returns the updated value for
pending_bytes. Thanks,

Alex

> > > > + * d. Read data_size bytes of data from (region + data_offset) from the
> > > > + * migration region.
> > > > + * e. Process the data.
> > > > + * f. Read pending_bytes, which indicates that the data from the previous
> > > > + * iteration has been read. If pending_bytes > 0, go to step b.
> > > > + *
> > > > + * If an error occurs during the above sequence, the vendor driver can return
> > > > + * an error code for next read() or write() operation, which will terminate the
> > > > + * loop. The user application should then take the next necessary action, for
> > > > + * example, failing migration or terminating the user application.
> > > > + *
> > > > + * The user application can transition from the _SAVING|_RUNNING
> > > > + * (pre-copy state) to the _SAVING (stop-and-copy) state regardless of the
> > > > + * number of pending bytes. The user application should iterate in _SAVING
> > > > + * (stop-and-copy) until pending_bytes is 0.
> > > > + *
> > > > + * The sequence to be followed while _RESUMING device state is as follows:
> > > > + * While data for this device is available, repeat the following steps:
> > > > + * a. Read data_offset from where the user application should write data.
> > > > + * b. Write migration data starting at the migration region + data_offset for
> > > > + * the length determined by data_size from the migration source.
> > > > + * c. Write data_size, which indicates to the vendor driver that data is
> > > > + * written in the migration region. Vendor driver should apply the
> > > > + * user-provided migration region data to the device resume state.
> > > > + *
> > > > + * For the user application, data is opaque. The user application should write
> > > > + * data in the same order as the data is received and the data should be of
> > > > + * same transaction size at the source.
> > > > + */
> > > > +
> > > > +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_IS_ERROR(state) \
> > > > + ((state & VFIO_DEVICE_STATE_MASK) == (VFIO_DEVICE_STATE_SAVING | \
> > > > + VFIO_DEVICE_STATE_RESUMING))
> > > > +
> > > > +#define VFIO_DEVICE_STATE_SET_ERROR(state) \
> > > > + ((state & ~VFIO_DEVICE_STATE_MASK) | VFIO_DEVICE_SATE_SAVING | \
> > > > + VFIO_DEVICE_STATE_RESUMING)
> > > > +
> > > > + __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
> > > > --
> > > > 2.7.0
> > > >
> > >
> >
>