Re: [RFC PATCH 1/5] virtio: introduce SUSPEND bit in device status

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 8/16/2023 9:58 AM, Jason Wang wrote:
> On Tue, Aug 15, 2023 at 6:48âPM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:
> > On 8/15/2023 8:37 AM, Jason Wang wrote:
> > > On Tue, Aug 15, 2023 at 8:26âAM Jason Wang <jasowang@redhat.com> wrote:
> > > > On Mon, Aug 14, 2023 at 7:29âPM Zhu Lingshan <lingshan.zhu@intel.com> wrote:
> > > > > This patch introudces a new status bit in the device status: SUSPEND.
> > > > >
> > > > > This SUSPEND bit can be used by the driver to suspend a device,
> > > > > in order to stablize the device states and virtqueue states.
> > > > >
> > > > > Its main use case is live migration.
> > > > I think patch 3 needs to be squashed into this one.
> > Patch 3 mentions Virtqueue State that is introduced in patch 2,
> Ok, so let's reorder the let patch 2 come first.
OK, I can work on that

Thanks
> Thanks
>
> > so I think patch 3 should placed after patch 2.
> >
> > Thanks
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > > Signed-off-by: Eugenio PÃrez <eperezma@redhat.com>
> > > > > Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> > > > > ---
> > > > >    content.tex | 18 ++++++++++++++++++
> > > > >    1 file changed, 18 insertions(+)
> > > > >
> > > > > diff --git a/content.tex b/content.tex
> > > > > index 0a62dce..1bb4401 100644
> > > > > --- a/content.tex
> > > > > +++ b/content.tex
> > > > > @@ -47,6 +47,9 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
> > > > >    \item[DRIVER_OK (4)] Indicates that the driver is set up and ready to
> > > > >      drive the device.
> > > > >
> > > > > +\item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, indicates that the
> > > > > +  device has been suspended by the driver.
> > > > > +
> > > > >    \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has experienced
> > > > >      an error from which it can't recover.
> > > > >    \end{description}
> > > > > @@ -73,6 +76,10 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
> > > > >    recover by issuing a reset.
> > > > >    \end{note}
> > > > >
> > > > > +The driver MUST NOT set SUSPEND if FEATURES_OK is not set.
> > > > > +
> > > > > +When set SUSPEND, the driver MUST re-read \field{device status} to ensure the SUSPEND bit is set.
> > > > > +
> > > > >    \devicenormative{\subsection}{Device Status Field}{Basic Facilities of a Virtio Device / Device Status Field}
> > > > >
> > > > >    The device MUST NOT consume buffers or send any used buffer
> > > > > @@ -82,6 +89,13 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
> > > > >    that a reset is needed.  If DRIVER_OK is set, after it sets DEVICE_NEEDS_RESET, the device
> > > > >    MUST send a device configuration change notification to the driver.
> > > > >
> > > > > +The device MUST ignore SUSPEND if FEATURES_OK is not set.
> > > > Is there any value to allow SUSPEND to be set without DRIVER_OK?
> > If live migration happens when the guest initializing the device,
> > maybe the guest driver has already configured some virtqueues or
> > modified the config space. At that point, We should allow to suspend
> > the device, because the hypervisor should not reset the device at
> > that moment.
> >
> > SUSPEND requires FEATURES_OK because VIRTIO_F_SUSPEND has to be negotiated.
> > > > > +
> > > > > +The deivce MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated.
> > > > typo.
> > Yes
> > > > Thanks
> > > Btw, it's not clear to me if driver is allowed to clear this bit.
> > Patch 1 mentions that DRIVER_OK clears SUSPEND, so that the driver
> > can resume the device if LM fails.
> >
> > Thanks
> > > Thanks
> > >
> > > > > +
> > > > > +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device MUST clear SUSPEND
> > > > > +and resumes operation upon DRIVER_OK.
> > > > > +
> > > > >    \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature Bits}
> > > > >
> > > > >    Each virtio device offers all the features it understands.  During
> > > > > @@ -872,6 +886,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> > > > >           \ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for
> > > > >           handling features reserved for future use.
> > > > >
> > > > > +  \item[VIRTIO_F_SUSPEND(41)] This feature indicates that the driver can
> > > > > +   SUSPEND the device.
> > > > > +   See \ref{sec:Basic Facilities of a Virtio Device / Device Status Field}.
> > > > > +
> > > > >    \end{description}
> > > > >
> > > > >    \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > > > > --
> > > > > 2.35.3