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

Kirti Wankhede
2018-10-17T20:47:19Z

On 10/17/2018 4:04 AM, Alex Williamson wrote:
> On Tue, 16 Oct 2018 23:42:35 +0530
> 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 quite a bit more than an "INFO" ioctl.
>
>> + * 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
>
> Of course the region migration region can describe itself as being used
> for migration, so this is unnecessary. The presence of that region
> could also negate the need for a probe.
>

Yes, that can be done.

>> + *
>> + * 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
>
> Valid states are the enum defined below, correct?
>

Yes, that's correct.

> Does setting STOPNCOPY_ACTIVE stop any state change of the device or is
> that expected to happen through other means?
>

_PRECOPY_ACTIVE means vCPUs are still running, so VFIO device should
still remain active.
_STOPNCOPY_ACTIVE means vCPUs are not running and device should also be
stopped and copy device's state.

> What are the allowable state transitions?
>

Normal VM running case:
_NONE -> _RUNNING

In case of live migration, at source:
_RUNNING -> _SETUP -> _PRECOPY_ACTIVE -> _STOPNCOPY_ACTIVE ->
_SAVE_COMPLETED

at destination:
_NONE -> _SETUP -> _RESUME -> _RESUME_COMPLETE -> _RUNNING

In save VM case:
_RUNNING -> _SETUP -> _STOPNCOPY_ACTIVE -> _SAVE_COMPLETED

In case of resuming VM from saved state:
_NONE -> _SETUP -> _RESUME -> _RESUME_COMPLETE -> _RUNNING

_FAILED or _CANCELLED can happen in any state.

> How many bits in flags is a user allowed to set at once?
>

One bit at a time. Probably, I should use enum for flags rather than bits.

>> + * 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.
>
> What's the significance of the provided threshold, are the pending
> bytes limited to threshold size? It makes me nervous to define a
> kernel API in terms of the internal API of a userspace program that can
> change. I wonder if it makes sense to define these in terms of the
> state of the devices, pending initial data, runtime data, post-offline
> data.
>

Threshold is required, because that will tell size in bytes that user
space application buffer can accommodate. Driver can copy data less than
threshold, but copying data more than threshold doesn't make sense
because user space application won't be able to copy that extra data and
that data might get overwritten or lost.

>> + *
>> + * 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
>
> Does the data the user gets here depend on the device state set
> earlier? For example the user gets pending_precopy_only data while
> PRECOPY_ACTIVE is the device state and pending_postcopy_only data
> in STOPNCOPY_ACTIVE? The user should continue to call GET_BUFFER
> in a given state until the associated pending field reaches zero?
> Jumping between the region and ioctl is rather awkward.
>

User gets pending_precopy_only data when device is in PRECOPY_ACTIVE
state, but each time when user calls GET_BUFFER, pending bytes might
change.
VFIO device's driver is producer of data and user/QEMU is consumer of
data. In pre-copy phase, when vCPUs are still running, driver will try
to accumulate as much data as possible in this phase, but vCPUs are
running and user of that device/application in guest is actively using
that device, then there are chances that during next iteration of
GET_BUFFER, driver might have more data.
Even in case of STOPNCOPY_ACTIVE state of device, driver can start
sending data in parts while a thread in vendor driver can still generate
data after device has halted, producer and consumer can run in parallel.
So User has to call GET_BUFFER until pending bytes are returned 0.

>> + *
>> + * 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.
>> + *
>> + * 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 */
>
> We'd need to swap device_state and size for alignment or else this
> struct could vary by compiler.
>

Ok. Thanks for pointing that out. I'll change that.

>> + __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[];
>> +};
>> +
>> +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,
>> +};
>> +
>> +#define VFIO_DEVICE_MIGRATION_INFO _IO(VFIO_TYPE, VFIO_BASE + 17)
>> +
>
> I'm not entirely sure what this ioctl buys us that we can't do with a
> region alone. For example we could define a migration region as:
>
> struct vfio_region_migration {
> __u32 device_state;
> __u32 data_offset;
> __u64 data_size;
> __u64 pending_pre;
> __u64 pending_runtime;
> __u64 pending_post;
> __u64 bytes_available;
> __u64 bytes_written;
> };
>
> A sparse mmap capability in the region could define whether the data
> area supports mmap, the above defined registers would only have
> read/write access. The user can write device_state the same as the
> ioctl would set it. The data_offset and size describe the offset in
> the region where migration data is relayed and the size of that area.
> Pending fields are as you have above, though I've omitted the threshold
> because I don't understand it. The user reading bytes_available is the
> same as calling GET_BUFFER, bytes_written the same as SET_BUFFER.

Yes, this could be done. Read/write on structure will be same as ioctl,
i.e. block till expected operation is done.

> I've
> left out the dirty bitmap here, as it's a little less obvious to me how
> it would work. It could be a stand-alone ioctl, it could be
> implemented as another region, or it could simply be a different offset
> within this region. The dirty bitmap start address could simply be
> calculated as the offset into that region where a pread begins and the
> extent is the size of that pread. With a bit per page, we're only
> looking at a 32MB region per 1TB of guest physical address space, so it
> doesn't seem too terrible even if guest memory is sparse. Maybe the
> extremes are still problematic though, but if it were part of the same
> region above we could solve that with another register written by the
> user to indicate the base offset of the dirty bitmap window.

Generally guest memory is sparse and log_sync is called for each
MemoryRegionSection. Same region can be used to get dirty page bitmap if
we are going to move to using sparse region.
I think here you are thinking of worst case where a VM with 1TB of
memory have one MemoryRegionSection of 1TB.

> For
> example if the window is 32MB and page size 4k then it can index 1TB
> and the user would write 0 (default) to the register for pfns from
> 0 to (1TB - 1), 1 for pfns from 1TB to (2TB - 1), etc.
>

Sorry I didn't get this example. By window here, do you meant sparse
region size?

> I don't see that this interface does anything to address the
> compatibility concerns that were discussed in the previous iterations
> from Intel though, nor is it documented exactly where and how a vendor
> driver would indicate a migration failure if it had its own internal
> consistency or compatibility checks fail. Thanks,
>

If vendor driver finds any inconsistency in data or if any operation is
causing failure, vendor driver should return error to the corresponding
ioctl. ioctl failure is treated as failure of the particular device. If
one device returns failure during migration process, migration is
terminated and VM at source resumes.

This RFC is mainly focused on core logic of live migration which include
pre-copy, stop-n-copy and resume phases. We can still continue
discussion on compatibility concerns, that could be either separate
change or can be merged in core logic. But I would also like to get some
focus on core logic as well.

Thanks,
Kirti