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

[Jason Wang <jasowang@redhat.com>]

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.

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
> >>>
>