Re: [PATCH 2/5] virtio: introduce SUSPEND bit in device status

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

On 9/14/2023 7:34 PM, Michael S. Tsirkin wrote:
> On Wed, Sep 06, 2023 at 04:16:34PM +0800, Zhu Lingshan wrote:
> > This patch introduces 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 stabilize the device states and virtqueue states.
> >
> > Its main use case is live migration.
> >
> > Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > Signed-off-by: Eugenio PÃrez <eperezma@redhat.com>
> > ---
> >   content.tex | 31 +++++++++++++++++++++++++++++++
> >   1 file changed, 31 insertions(+)
> >
> > diff --git a/content.tex b/content.tex
> > index 0e492cd..0fab537 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 SHOULD NOT set SUSPEND if FEATURES_OK is not set.
> > +
> > +When setting 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 +89,26 @@ \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.
> > +
> > +The device MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated.
> why? let's just forbid driver from setting it.
OK
> > +
> > +The device SHOULD allow settings to \field{device status} even when SUSPEND is set.
> > +
> > +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device SHOULD clear SUSPEND
> > +and resumes operation upon DRIVER_OK.
> > +
> sorry what?
In case of a failed or cancelled Live Migration, the device needs to 
resume operation.
However the spec forbids the driver to clear a device status bit, so 
re-writing
DRIVER_OK is expected to clear SUSPEND and the device resume operation.
> > +If VIRTIO_F_SUSPEND is negotiated, when the driver sets SUSPEND,
> > +the device SHOULD perform the following actions before presenting SUSPEND bit in the \field{device status}:
> > +
> > +\begin{itemize}
> > +\item Stop consuming buffers of any virtqueues and mark all finished descritors as used.
> > +\item Wait until all descriptors that being processed to finish and mark them as used.
> > +\item Flush all used buffer and send used buffer notifications to the driver.
> flush how?
This is device-type-specific, and we will include tracking inflight 
descriptors(buffers) in V2.
> > +\item Record Virtqueue State of each enabled virtqueue, see section \ref{sec:Virtqueues / Virtqueue State}
>
> record where?
This is transport specific, for PCI, patch 5 introduces two new fields 
for avail and used state
> > +\item Pause its operation except \field{device status} and preserve configurations in its Device Configuration Space, see \ref{sec:Basic Facilities of a Virtio Device / Device Configuration Space}
> pause in what sense? completely?  this does not seem realistic.
> e.g. pci express link has to stay active or device will die.
only pause virtio, I will rephrase the sentence as "pause its virtio 
operation".
Others like PCI link in the example is out of the spec and we don't need
to migrate them.
> also, presumably here it is except a bunch of other fields.
> e.g. what about queue select and all related queue fields?
For now they are forbidden.

As SiWei suggested, we will introduce a new feature bit to control whether
allowing resetting a VQ after SUSPEND. We can use more feature bits if
there are requirements to perform anything after SUSPEND. But for now
they are forbidden.
> > +\end{itemize}
> > +
> >   \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature Bits}
> >   
> >   Each virtio device offers all the features it understands.  During
> > @@ -937,6 +964,10 @@ \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(42)] 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