allow concurrent dirty bitmap get

[jlevon]

Use atomic operations to allow concurrent bitmap updates with VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP operations.

Dirtying clients can race against each other, so we must use atomic or when marking dirty: we do this byte-by-byte.

When reading the dirty bitmap, we must be careful to not race and lose any set bits within the same byte. If we miss an update, we'll catch it the next time around, presuming that before the final pass we'll have quiesced all I/O.

Signed-off-by: John Levon <john.levon@nutanix.com>

[jlevon]

could read a little cleaner

you could well be correct, but I've struggled to do so - do you have a suggestion?

[jlevon]

in the meantime I will merge