Re: [RFC PATCH v1 1/4] VFIO KABI for migration interface

Kirti Wankhede
2018-10-17T21:17:22Z

On 10/17/2018 3:39 PM, Dr. David Alan Gilbert wrote:
> * Kirti Wankhede (kwankhede@nvidia.com) wrote:
>> - Added vfio_device_migration_info structure to use interact with vendor
>> driver.
>> - Different flags are used to get or set migration related information
>> from/to vendor driver.
>> Flag VFIO_MIGRATION_PROBE: To query if feature is supported
>> Flag VFIO_MIGRATION_GET_REGION: To get migration region info
>> Flag VFIO_MIGRATION_SET_STATE: To convey device state in vendor driver
>> Flag VFIO_MIGRATION_GET_PENDING: To get pending bytes yet to be migrated
>> from vendor driver
>> Flag VFIO_MIGRATION_GET_BUFFER: On this flag, vendor driver should write
>> data to migration region and return number of bytes written in the region
>> Flag VFIO_MIGRATION_SET_BUFFER: In migration resume path, user space app
>> writes to migration region and communicates it to vendor driver with
>> this ioctl with this flag.
>> Flag VFIO_MIGRATION_GET_DIRTY_PFNS: Get bitmap of dirty pages from vendor
>> driver from given start address
>>
>> - Added enum for possible device states.
>>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>> ---
>> linux-headers/linux/vfio.h | 91 ++++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 91 insertions(+)
>>
>> diff --git a/linux-headers/linux/vfio.h b/linux-headers/linux/vfio.h
>> index 3615a269d378..8e9045ed9aa8 100644
>> --- a/linux-headers/linux/vfio.h
>> +++ b/linux-headers/linux/vfio.h
>> @@ -602,6 +602,97 @@ struct vfio_device_ioeventfd {
>>
>> #define VFIO_DEVICE_IOEVENTFD _IO(VFIO_TYPE, VFIO_BASE + 16)
>>
>> +/**
>> + * VFIO_DEVICE_MIGRATION_INFO - _IOW(VFIO_TYPE, VFIO_BASE + 17,
>> + * struct vfio_device_migration_info)
>
> This is a little odd; what you really have here is 7 different
> operations that can be performed; they're independent operations all
> relating to part of migration; so instead of a 'flag' it's more of an
> operation type, and there's no reason to make them individual bit flags
> - for edxample there's no reason you'd want to OR together
> MIGRATION_GET_REGION and MIGRATION_GET_PENDING - they're just
> independent.
> (Similarly you could just make them 7 ioctls)
>

Right, all flags are independent commands. But I tried to use one ioctl
number.

> I guess what you're trying to do here is a direct 1-1 mapping of qemu's
> struct SaveVMHandlers interface to devices.
> That's not necessarily a bad idea, but remember it's not a stable
> interface, so QEMU will change it over time and you'll have to then
> figure out how to shim these interfaces to it, so it's worth keeping
> that in mind, just in case you can make these interfaces more general.
>

Alex suggested using sparse region instead of ioctl, I'm making note of
your point to define structures when implementing this interface using
sparse mmap region.

>> + * Flag VFIO_MIGRATION_PROBE:
>> + * To query if feature is supported
>> + *
>> + * Flag VFIO_MIGRATION_GET_REGION:
>> + * To get migration region info
>> + * region_index [output] : region index to be used for migration region
>> + * size [output]: size of migration region
>> + *
>> + * Flag VFIO_MIGRATION_SET_STATE:
>> + * To set device state in vendor driver
>> + * device_state [input] : User space app sends device state to vendor
>> + * driver on state change
>> + *
>> + * Flag VFIO_MIGRATION_GET_PENDING:
>> + * To get pending bytes yet to be migrated from vendor driver
>> + * threshold_size [Input] : threshold of buffer in User space app.
>> + * pending_precopy_only [output] : pending data which must be migrated in
>> + * precopy phase or in stopped state, in other words - before target
>> + * vm start
>> + * pending_compatible [output] : pending data which may be migrated in any
>> + * phase
>> + * pending_postcopy_only [output] : pending data which must be migrated in
>> + * postcopy phase or in stopped state, in other words - after source
>> + * vm stop
>> + * Sum of pending_precopy_only, pending_compatible and
>> + * pending_postcopy_only is the whole amount of pending data.
>> + *
>> + * Flag VFIO_MIGRATION_GET_BUFFER:
>> + * On this flag, vendor driver should write data to migration region and
>> + * return number of bytes written in the region.
>> + * bytes_written [output] : number of bytes written in migration buffer by
>> + * vendor driver
>> + *
>> + * Flag VFIO_MIGRATION_SET_BUFFER
>> + * In migration resume path, user space app writes to migration region and
>> + * communicates it to vendor driver with this ioctl with this flag.
>> + * bytes_written [Input] : number of bytes written in migration buffer by
>> + * user space app.
>
> I guess we call getbuffer/setbuffer multiple times. Is there anything
> that needs to be passed to get the data to line up (e.g. offset into the
> data)
>

I think, vendor driver can keep track of offset as it knows what data
copied and what is remaining.

>> + * Flag VFIO_MIGRATION_GET_DIRTY_PFNS
>> + * Get bitmap of dirty pages from vendor driver from given start address.
>> + * start_addr [Input] : start address
>> + * pfn_count [Input] : Total pfn count from start_addr for which dirty
>> + * bitmap is requested
>> + * dirty_bitmap [Output] : bitmap memory allocated by user space
>> + * application, vendor driver should return the bitmap with bits set
>> + * only for pages to be marked dirty.
>> + * Return: 0 on success, -errno on failure.
>> + */
>> +
>> +struct vfio_device_migration_info {
>> + __u32 argsz;
>> + __u32 flags;
>> +#define VFIO_MIGRATION_PROBE (1 << 0)
>> +#define VFIO_MIGRATION_GET_REGION (1 << 1)
>> +#define VFIO_MIGRATION_SET_STATE (1 << 2)
>> +#define VFIO_MIGRATION_GET_PENDING (1 << 3)
>> +#define VFIO_MIGRATION_GET_BUFFER (1 << 4)
>> +#define VFIO_MIGRATION_SET_BUFFER (1 << 5)
>> +#define VFIO_MIGRATION_GET_DIRTY_PFNS (1 << 6)
>> + __u32 region_index; /* region index */
>> + __u64 size; /* size */
>> + __u32 device_state; /* VFIO device state */
>> + __u64 pending_precopy_only;
>> + __u64 pending_compatible;
>> + __u64 pending_postcopy_only;
>> + __u64 threshold_size;
>> + __u64 bytes_written;
>> + __u64 start_addr;
>> + __u64 pfn_count;
>> + __u8 dirty_bitmap[];
>> +};
>
> This feels like it should be separate types for the different calls.
>
> Also, is there no state to tell the userspace what version of migration
> data we have? What happens if I try and load a migration state
> from an older driver into a newer one, or the other way around,
> or into a destination card that's not the same as the source.
>

As I mentioned in other reply, this RFC is mainly focused on core logic
of live migration which include implementation for pre-copy, stop-n-copy
and resume phases, defining device states.

>> +enum {
>> + VFIO_DEVICE_STATE_NONE,
>> + VFIO_DEVICE_STATE_RUNNING,
>> + VFIO_DEVICE_STATE_MIGRATION_SETUP,
>> + VFIO_DEVICE_STATE_MIGRATION_PRECOPY_ACTIVE,
>> + VFIO_DEVICE_STATE_MIGRATION_STOPNCOPY_ACTIVE,
>> + VFIO_DEVICE_STATE_MIGRATION_SAVE_COMPLETED,
>> + VFIO_DEVICE_STATE_MIGRATION_RESUME,
>> + VFIO_DEVICE_STATE_MIGRATION_RESUME_COMPLETED,
>> + VFIO_DEVICE_STATE_MIGRATION_FAILED,
>> + VFIO_DEVICE_STATE_MIGRATION_CANCELLED,
>> +};
>
> That's an interesting merge of QEMU's run-state and it's migration
> state.
>
> You probably need to define which transitions you take as legal.
>

In reply to Alex, I had listed down allowable state transitions.

Thanks,
Kirti

> Dave
>
>> +
>> +#define VFIO_DEVICE_MIGRATION_INFO _IO(VFIO_TYPE, VFIO_BASE + 17)
>> +
>> /* -------- API for Type1 VFIO IOMMU -------- */
>>
>> /**
>> --
>> 2.7.0
>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>