Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

[Jason Wang <jasowang@redhat.com>]

On Tue, Oct 31, 2023 at 11:27âAM Parav Pandit <parav@nvidia.com> wrote:
>
>
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Tuesday, October 31, 2023 7:13 AM
> >
> > On Mon, Oct 30, 2023 at 9:21âPM Parav Pandit <parav@nvidia.com> wrote:
> > >
> > > During a device migration flow (typically in a precopy phase of the
> > > live migration), a device may write to the guest memory. Some
> > > iommu/hypervisor may not be able to track these written pages.
> > > These pages to be migrated from source to destination hypervisor.
> > >
> > > A device which writes to these pages, provides the page address record
> > > of the to the owner device. The owner device starts write recording
> > > for the device and queries all the page addresses written by the
> > > device.
> > >
> > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/176
> > > Signed-off-by: Parav Pandit <parav@nvidia.com>
> > > Signed-off-by: Satananda Burla <sburla@marvell.com>
> > > ---
> > > changelog:
> > > v1->v2:
> > > - addressed comments from Michael
> > > - replaced iova with physical address
> > > ---
> > >  admin-cmds-device-migration.tex | 15 +++++++++++++++
> > >  1 file changed, 15 insertions(+)
> > >
> > > diff --git a/admin-cmds-device-migration.tex
> > > b/admin-cmds-device-migration.tex index ed911e4..2e32f2c 100644
> > > --- a/admin-cmds-device-migration.tex
> > > +++ b/admin-cmds-device-migration.tex
> > > @@ -95,6 +95,21 @@ \subsubsection{Device Migration}\label{sec:Basic
> > > Facilities of a Virtio Device /  The owner driver can discard any
> > > partially read or written device context when  any of the device migration flow
> > should be aborted.
> > >
> > > +During the device migration flow, a passthrough device may write data
> > > +to the guest virtual machine's memory, a source hypervisor needs to
> > > +keep track of these written memory to migrate such memory to destination
> > hypervisor.
> > > +Some systems may not be able to keep track of such memory write
> > > +addresses at hypervisor level. In such a scenario, a device records
> > > +and reports these written memory addresses to the owner device. The
> > > +owner driver enables write recording for one or more physical address
> > > +ranges per device during device migration flow. The owner driver
> > > +periodically queries these written physical address records from the device.
> >
> > I wonder how PA works in this case. Device uses untranslated requests so it can
> > only see IOVA. We can't mandate ATS anyhow.
> Michael suggested to keep the language uniform as PA as this is ultimately what the guest driver is supplying during vq creation and in posting buffers as physical address.

This seems to need some work. And, can you show me how it can work?

1) e.g if GAW is 48 bit, is the hypervisor expected to do a bisection
of the whole range?
2) does the device need to reserve sufficient internal resources for
logging the dirty page and why (not)?
3) DMA is part of the transport, it's natural to do logging there, why
duplicate efforts in the virtio layer? I can't see how it can compete
with the functionality that is provided by the platform. And what's
more, we can't assume virtio is the only device that is used by the
guest.

Thanks