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Subject: Re: [PATCH 2/5] virtio: introduce SUSPEND bit in device status
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 Pérez <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. > + > +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? > +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? > +\item Record Virtqueue State of each enabled virtqueue, see section \ref{sec:Virtqueues / Virtqueue State} record where? > +\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. also, presumably here it is except a bunch of other fields. e.g. what about queue select and all related queue fields? > +\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
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