Re: [PATCH v7 07/10] vfio: Extend the device migration protocol with PRE_COPY

[Jason Gunthorpe]

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