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

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

On 9/18/2023 10:56 AM, Zhu, Lingshan wrote:
> On 9/15/2023 7:10 PM, Michael S. Tsirkin wrote:
> > 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 PÃrez <eperezma@redhat.com>
> > > > > ---
> > > > > ÂÂ content.tex | 31 +++++++++++++++++++++++++++++++
> > > > > ÂÂ 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
> > > > > ÂÂ \item[DRIVER_OK (4)] Indicates that the driver is set up and 
> > > > > ready to
> > > > > ÂÂÂÂ drive the device.
> > > > > +\item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, 
> > > > > indicates that the
> > > > > +Â device has been suspended by the driver.
> > > > > +
> > > > > ÂÂ \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has 
> > > > > experienced
> > > > > ÂÂÂÂ an error from which it can't recover.
> > > > > ÂÂ \end{description}
> > > > > @@ -73,6 +76,10 @@ \section{\field{Device Status} 
> > > > > Field}\label{sec:Basic Facilities of a Virtio Dev
> > > > > ÂÂ recover by issuing a reset.
> > > > > ÂÂ \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.
> > > > > +
> > > > > ÂÂ \devicenormative{\subsection}{Device Status Field}{Basic 
> > > > > Facilities of a Virtio Device / Device Status Field}
> > > > > ÂÂ 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
> > > > > ÂÂ 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.
> The spec says: Set the DRIVER_OK status bit. At this point the device 
> is âliveâ.
>
> So semantically DRIVER_OK can bring the device to live even from SUSPEND.
>
> In the implementation, the device can check whether SUSPEND is set, then
> decide what to do. Just don't ignore DRIVER_OK if it is already
> set, and the driver should not clear a device status bit.
>
> > > > > +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.
> vq states filed can work with queue_select like other vq fields.
> I will document this in the comment.
> > > > > +\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?
> Then how about we say: pause operation in both data-path and 
> control-path?
>
> Or do you have any suggestion?
> > > 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.
> a new feature bit: VIRTIO_F_RING_SUSPEND_RESET. If this feature
> bit has been negotiated then the device allow reset a vq after SUSPEND.
Hi Michael,

Rethink of this, as you suggested before, In V2, I will forbid resetting
VQs after SUSPEND.

Thanks
> > > > > +\end{itemize}
> > > > > +
> > > > > ÂÂ \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}
> > > > > ÂÂÂÂÂÂ \ref{devicenormative:Basic Facilities of a Virtio Device / 
> > > > > Feature Bits} for
> > > > > ÂÂÂÂÂÂ handling features reserved for future use.
> > > > > +Â \item[VIRTIO_F_SUSPEND(42)] This feature indicates that the 
> > > > > driver can
> > > > > +ÂÂ SUSPEND the device.
> > > > > +ÂÂ See \ref{sec:Basic Facilities of a Virtio Device / Device 
> > > > > Status Field}.
> > > > > +
> > > > > ÂÂ \end{description}
> > > > > ÂÂ \drivernormative{\section}{Reserved Feature Bits}{Reserved 
> > > > > Feature Bits}
> > > > > --
> > > > > 2.35.3