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Subject: Re: [PATCH 2/5] virtio: introduce SUSPEND bit in device status
On Fri, Sep 15, 2023 at 10:57:33AM +0800, Zhu, Lingshan wrote: > > > 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 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. > 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. No, DRIVER_OK is already set. Setting a bit that is already set should not have side effects. In fact auto-clearing suspend is problematic too. > > > > > +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 they clearly can't store state for all vqs, these are just two 16 bit fields. > > > > > +\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". that is vague too. for example what happens to link state of a networking device? > 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. I don't know how this means, but whatever. you need to make all this explicit though. > > > > > +\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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