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Subject: Re: [virtio-comment] Re: [RFC PATCH 4/5] virtqueue: constraints for virtqueue state
On 9/7/2023 4:09 PM, Eugenio Perez Martin wrote:
On Tue, Sep 5, 2023 at 11:08âAM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:On 8/21/2023 5:26 PM, Eugenio Perez Martin wrote:On Fri, Aug 18, 2023 at 11:44âAM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:On 8/17/2023 11:19 PM, Eugenio Perez Martin wrote:On Tue, Aug 15, 2023 at 1:30âPM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:On 8/15/2023 8:34 AM, Jason Wang wrote:On Mon, Aug 14, 2023 at 7:29âPM Zhu Lingshan <lingshan.zhu@intel.com> wrote:This commit specifies the constraints of the virtqueue state, and the actions should be taken by the device when SUSPEND and DRIVER_OK is set Signed-off-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com> --- content.tex | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) diff --git a/content.tex b/content.tex index 43bd5de..f6ac581 100644 --- a/content.tex +++ b/content.tex @@ -587,6 +587,37 @@ \subsection{\field{Used State} Field} See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}. +\drivernormative{\subsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Virtqueue State} + +If VIRTIO_F_QUEUE_STATE has been negotiated, the driver MUST set SUSPEND in \field{device status} +first before getting or setting Virtqueue State of any virtqueues.I don't get why this is a must. It could be useful for debugging.To avoid race conditions with the device and make the device implementation easier+ +If VIRTIO_F_QUEUE_STATE has been negotiaged but VIRTIO_RING_F_PACKED not been negotiated,typoyes+the driver MUST NOT access \field{Used State} of any virtqueues, it should use the +used index in the used ring. + +\devicenormative{\subsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Virtqueue State} + +If VIRTIO_F_QUEUE_STATE has been negotiated but SUSPEND is not set in \field{device status}, +the device MUST ignore any accesses against Virtqueue State of any virtqueues.Btw, do we need to clarify the behavior of ring reset after suspending?I think once suspended, the device should ignore resetting a queueActually shadow virtqueue could benefit from the ability to change vq properties (addresses) while the device is suspended, and then just resume it. I've been told that ring reset is overkill for that.If ring reset is overkill, is SUSPEND even more overkill?It depends on the cost of recreating the vq in the device I think. But it has more to do with *what* is changed in the vq, as it seems some parameters (vq size) has more impact than others like vq address. The way to stop the device does not affect, but ring reset offers the possibility of change all of the parameters already. Adding Si-Wei and Dragos here, as they pointed it out in the virtio-networking upstream meeting.But probably it is better to address it on top, with another feature flag.I think if we want to changing the vq properties, there must be a mechanism to stop the queue then resume the queue. How about allow setting queue_enable = 0 to stop it and =1 to resume and force it reinitialize?Yes, I think that is better suited. But maybe this is better to be added on top, so we maintain this series small.Hi Eugenio, I have a second thought while implementing above queue_enable = 0, it doesn't provide more advantages over queue_reset: 1) queue_reset can help to stop a queue and the vq properties can be reconfigured during queue_reset --> queue_enable. 2) once the driver sees SUSPEND presented by the device, it assume the device states and vq states are stable, at that point the driver can read reliable device configurations. So vq reset should be ignored once SUSPEND is present and if we implement queue stop, it should be ignored too when SUSPEND.The relation between SUSPEND and ring_reset needs to be described in this series, yes. This is a good start, but I'm not sure if this one meets all the requirements for SW assisted live migration. We can always add new feature flags to define a different interaction in the future, like for devices that can support the change of vq attributes in the suspend. To not steal the merit, this idea was proposed by Si-Wei in a recent virtio-networking meeting.
If so, we even don't need a new feature bit. We can just allow resetting vqs after the device presenting SUSPEND. The device presenting SUSPEND indicates that the device config spaceis stabilized at that moment, ready for the driver to fetch fields data there.
Then the driver is allowed to reset, re-config and re-enable the vqs. The only requirement is: The driver is responsible for maintain the integrity and validity of the config space fields, because the device is ready-only to the config space at that moment(SUSPEND-ed) and the driver should be responsible for its actions, perform proper synchronizations, e.g., re-read. Does this work for you? Thanks
3) the device should only accept resetting a queue when !SUSPEND and the driver can flush the queue buffers before resetting it to avoid losing buffers, and we will have tracker for in-flight descriptors later. Any thoughts? ThanksThanks!Thanks Zhu Lingshan+ +When VIRTIO_F_QUEUE_STATE has been negotiated but VIRTIO_RING_F_PACKED is not, +the device MUST ignore any accesses against \field{Used State}. + +If VIRTIO_F_QUEUE_STATE has been negotiaged, the device MUST reset +the Virtqueue State of every virtqueue upon a reset.Need to define the meaning of "reset" this is important for packed virtqueue.I will remove this as Stefan suggested.+ +If VIRTIO_F_QUEUE_STATE and VIRTIO_RING_F_PACKED have been negotiaged, when SUSPEND is set, +the device MUST record the Virtqueue State of every enabled virtqueue +in \field{Available State} and \field{Used State} respectively, +and correspondingly restore the Virtqueue State of every enabled virtqueue +from \field{Avaiable State} and \field{Used State} when DRIVER_OK is set.We can just let the device report those states in any case then we don't need to care about those details, or did you see any blockers?Agree, I will add the definition of used_state of splitted vq in the next version ThanksThanks+ +If VIRTIO_F_QUEUE_STATE has been negotiated but VIRTIO_RING_F_PACKED has been not, when SUSPEND is set, +the device MUST record the available state of every enabled virtqueue in \field{Available State}, +and restore the available state of every enabled virtqueue from \field{Avaiable State} +when DRIVER_OK is set. + \input{admin.tex} \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation} -- 2.35.3This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. Subscribe: virtio-comment-subscribe@lists.oasis-open.org Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org List help: virtio-comment-help@lists.oasis-open.org List archive: https://lists.oasis-open.org/archives/virtio-comment/ Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists Committee: https://www.oasis-open.org/committees/virtio/ Join OASIS: https://www.oasis-open.org/join/
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