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

[Jason Gunthorpe]

On Wed, Mar 02, 2022 at 08:47:52PM -0700, Alex Williamson wrote:
> On Wed, 2 Mar 2022 20:05:28 -0400
> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> > 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.
> 
> But there's no requirement to read anything in PRE_COPY, so initial
> becomes indistinguishable from trailer and dirty doesn't exist.

It is still mandatory to read that data out, it doesn't matter if it
is read during PRE_COPY or STOP_COPY.

> > > "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'
> 
> 'initial precopy data-set'?

Sure

> We have no basis to make that assertion.  We've agreed that precopy can
> be used for nothing more than a compatibility test, so we could have a
> vGPU with a massive framebuffer and no ability to provide dirty
> tracking implement precopy only to include the entire framebuffer in
> the trailing STOP_COPY data set.  Per my understanding and the fact
> that we cannot enforce any heuristics regarding the size of the tailer
> relative to the pre-copy data set, I think the above strongly phrased
> sentence is necessary to understand the limitations of what this ioctl
> is meant to convey.  Thanks,

This is why abusing precopy for compatability is not a great idea. It
is OK for acc because its total state is tiny, but I would not agree
to a vGPU driver being merged working like you describe. It distorts
the entire purpose of PRE_COPY and this whole estimation mechanism.

The ioctl is intended to convey when to switch to STOP_COPY, and the
driver should provide a semantic where the closer the reported length
is to 0 then the faster the STOP_COPY will go.

Jason