On Wed, Mar 02, 2022 at 01:31:59PM -0700, Alex Williamson wrote: > > + * initial_bytes reflects the estimated remaining size of any initial mandatory > > + * precopy data transfer. When initial_bytes returns as zero then the initial > > + * phase of the precopy data is completed. Generally initial_bytes should start > > + * out as approximately the entire device state. > > What is "mandatory" intended to mean here? The user isn't required to > collect any data from the device in the PRE_COPY states. If the data is split into initial,dirty,trailer then mandatory means that first chunk. > "The vfio_precopy_info data structure returned by this ioctl provides > estimates of data available from the device during the PRE_COPY states. > This estimate is split into two categories, initial_bytes and > dirty_bytes. > > The initial_bytes field indicates the amount of static data available > from the device. This field should have a non-zero initial value and > decrease as migration data is read from the device. static isn't great either, how about just say 'minimum data available' > Userspace may use the combination of these fields to estimate the > potential data size available during the PRE_COPY phases, as well as > trends relative to the rate the device is dirtying it's internal > state, but these fields are not required to have any bearing relative > to the data size available during the STOP_COPY phase." That last is too strong. I would just drop starting at but. The message to communicate is the device should allow dirty_bytes to reach 0 during the PRE_COPY phases if everything is is idle. Which tells alot about how to calculate it. It is all better otherwise > > + * Drivers should attempt to return estimates so that initial_bytes + > > + * dirty_bytes matches the amount of data an immediate transition to STOP_COPY > > + * will require to be streamed. > > I think previous discussions have proven this false, we expect trailing > data that is only available in STOP_COPY, we cannot bound the size of > that data, and dirty_bytes is not intended to expose data that cannot > be retrieved during the PRE_COPY phases. Thanks, It was written assuming the stop_copy trailer is small. Thanks, Jason
