support for live migration

[tmakatos]

Implementing live migration requires providing a migration region with a VFIO region info type capability. Currently we only support the sparse mmap capablility so we don't require the user to create a VFIO capability header and chain with the sparse mmap capability; we simply receive sparse areas via lm_reg_info_t.mmap_areas during context creation time. This made sense in the past as there was no other VFIO region capability that we had to support, however this now needs to change. Rather than adding a new member to lm_reg_info_t for passing the VFIO region info type capability (along with mmap_areas), we should refactor the code and take a generic VFIO region capability, just like we do for the PCI capabilities (lm_cap_t). This way we'll instantly support new VFIO region capability that might be added in the future.

So first, we need to make this change. Second, we need to extend the client/server samples to use this code. Third, we can continue working on the actual live migration.

[tmakatos]

I've done some work for supporting live migration in libmuser, I've over simplified most of the code to make it work. There are many open issues, most importantly understanding how some regisiters in the migration region are supposed to work (more information in the code as FIXME and TODO).

[tmakatos]

Also, check VFIO live migration in QEMU, it might answer lots of questions regarding how registers are used etc.

[tmakatos]

I see that in hw/vfio/migration.c, as soon as the device transitions to the stop-and-copy phase (before migration data are actually copied), the device configuration state is stored. IIUC the client is responsible for saving/loading it, so no need to do anything in muser.

[tmakatos]

I received some clarifications on some question I have (https://marc.info/?l=kvm&m=160502788221980&w=2):

• During the stop-and-copy and stopped state the device should only handle migration-specific operations. Any other operation (e.g. vfio-user request) should be explicitly failed.
• Reading data_offset marks a new iteration, so re-reading pending_bytes and data_size is allowed.
• The pending_bytes register is volatile.
• Reading data_size out of sequence if undefined.
• data_offset and data_size are invariant during an iteration.

[tmakatos]

Fixed.