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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: 03 March 2022 15:21
> To: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Cc: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>;
> kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linux-crypto@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; cohuck@xxxxxxxxxx;
> mgurtovoy@xxxxxxxxxx; yishaih@xxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>; liulongfang <liulongfang@xxxxxxxxxx>; Zengtao (B)
> <prime.zeng@xxxxxxxxxxxxx>; Jonathan Cameron
> <jonathan.cameron@xxxxxxxxxx>; Wangzhou (B) <wangzhou1@xxxxxxxxxxxxx>
> Subject: Re: [PATCH v7 07/10] vfio: Extend the device migration protocol with
> PRE_COPY
> 
> On Thu, 3 Mar 2022 09:01:24 -0400
> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> > 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.
> 
> Not really, PRE_COPY -> RUNNING is a valid arc.
> 
> > > > > "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.
> 
> If it's an abuse, then let's not do it.  It was never my impression or intention
> that this was ok for acc only due to the minimal trailing data size.  My
> statement was that use of PRE_COPY for compatibility testing only had been a
> previously agreed valid use case of the original migration interface.
> 
> Furthermore the acc driver was explicitly directed not to indicate any degree
> of trailing data size in dirty_bytes, so while trailing data may be small for acc,
> this interface is explicitly not intended to provide any indication of trailing
> data size.  Thanks,

Just to clarify, so the suggestion here is not to use PRE_COPY for compatibility
check at all and have a different proper infrastructure for that later as Jason
suggested?

If so, I will remove this patch from this series and go back to the old revision
where we only have STOP_COPY and do the compatibility check during the final
load data operation.

Please let me know.

Thanks,
Shameer