Re: [virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 17, 2023 at 05:50:24PM +0800, Zhu, Lingshan wrote:
> 
> 
> On 11/16/2023 8:18 PM, Michael S. Tsirkin wrote:
> > On Thu, Nov 16, 2023 at 05:38:35PM +0800, Zhu, Lingshan wrote:
> > > 
> > > On 11/15/2023 7:52 PM, Michael S. Tsirkin wrote:
> > > > On Wed, Nov 15, 2023 at 04:42:56PM +0800, Zhu, Lingshan wrote:
> > > > > On 11/15/2023 4:05 PM, Michael S. Tsirkin wrote:
> > > > > > On Wed, Nov 15, 2023 at 03:59:16PM +0800, Zhu, Lingshan wrote:
> > > > > > > On 11/15/2023 3:51 PM, Michael S. Tsirkin wrote:
> > > > > > > > On Wed, Nov 15, 2023 at 12:05:59PM +0800, Zhu, Lingshan wrote:
> > > > > > > > > On 11/14/2023 4:27 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > On Tue, Nov 14, 2023 at 03:34:32PM +0800, Zhu, Lingshan wrote:
> > > > > > > > > > > > > So I can't
> > > > > > > > > > > > > believe it has good performance overall. Logging via IOMMU or using
> > > > > > > > > > > > > shadow virtqueue doesn't need any extra PCI transactions at least.
> > > > > > > > > > > > On the other hand they have an extra CPU cost.  Personally if this is
> > > > > > > > > > > > coming from a hardware vendor, I am inclined to trust them wrt PCI
> > > > > > > > > > > > transactions.  But anyway, discussing this at a high level theoretically
> > > > > > > > > > > > is pointless - whoever bothers with actual prototyping for performance
> > > > > > > > > > > > testing wins, if no one does I'd expect a back of a napkin estimate
> > > > > > > > > > > > to be included.
> > > > > > > > > > > if so, Intel has released productions implementing these interfaces years
> > > > > > > > > > > ago,
> > > > > > > > > > > see live migration in 4.1. IFCVF vDPA Implementation,
> > > > > > > > > > > https://doc.dpdk.org/guides-21.11/vdpadevs/ifc.html
> > > > > > > > > > > and
> > > > > > > > > > That one is based on shadow queue, right? Which I think this shows
> > > > > > > > > > is worth supporting.
> > > > > > > > > Yes, it is shadow virtqueue, I assume this is already mostly done,
> > > > > > > > > do you see any gaps we need to address in our series that we should work on?
> > > > > > > > > 
> > > > > > > > > Thanks
> > > > > > > > There were a ton of comments posted on your series.
> > > > > > > Hope I didn't miss anything. I see your latest comments are about vq states,
> > > > > > > as replied before, I think we can record the states by two le16 and the
> > > > > > > in-flight
> > > > > > > descriptor tracking facility.
> > > > > > I don't know why you need the le16. in-flight tracking should be enough.
> > > > > > And given it needs DMA I would try really hard to actually use
> > > > > > admin commands for this.
> > > > > we need to record the on-device avail_idx and used_idx, or
> > > > > how can the destination side know the device internal values.
> > > > Again you never documented what state the device is in so I can't really
> > > > say for sure.  But generally whenever a buffer is used the internal
> > > > values are written out to memory.
> > > This is the state of a virtqueue, in my series I have defined what is
> > > vq state in [PATCH V2 1/6] virtio: introduce virtqueue state,
> > > and give an example of PCI implementation.
> > > 
> > > In short, for split vq it is last_avail_idx and in-flight descriptors.
> > > 
> > > I humbly request an explicit list of missing gaps, so that I can improve my
> > > V3
> > > 
> > > Thanks
> > I don't know how to help you without resorting to writing it instead of
> > you, I sent 3 messages in response to that one patch alone. Your patch
> > just adds some bits here and there without much in the way of
> > documentation. Patch needs to explain what these things are and how do they
> > interact with VQ state in memory.
> Please allow me quote the series:
> +The available state field is two bytes of virtqueue state that is used by
> +the device to read the next available buffer. It is presented in the
> following format:
> +
> +\begin{lstlisting}
> +le16 last_avail_idx;
> +\end{lstlisting}
> +
> +The \field{last_avail_idx} field is the free-running available ring
> +index where the device will read the next available head of a
> +descriptor chain.

Next *after what*? The last used buffer? This is exactly the used index.

> 
> +When SUSPEND is set, the device MUST record the Available State of every
> enabled splited virtqueue
> +in \field{Available State} field,
> +and correspondingly restore the Available State of every enabled splited
> virtqueue
> +from \field{Available State} field when DRIVER_OK is set.
> +
> +The device SHOULD reset \field{Available State} field upon a device reset
> 
> I will add these contents in next series:
> 1) vq states refer to the device internal states, so have to record and
> restore them
> 2) in-flight descriprots tracking.
> 
> Not sure whether I should describe how in-guest-memory vq config migrated,
> because they migrated with guest memory.
> > 
> > 
> > But besides, Parav needs to do exactly the same too. So why don't you
> > let Parav do the work on this and then later just add a small interface
> > to send admin commands through VF itself? Looks like this will be good
> I think this is the topic: shall we process admin cmds in config space.....
> > enough for VDPA. Meanwhile I feel your energy would be better spend
> > working on transport vq which no one else is working on.
> I can start rework on transport vq next week.
> 
> Thanks
> > 
> > 
> > 
> > 
> > > > > > > For this shadow virtqueue, do you think I should address this in my V4?
> > > > > > > Like saying: acknowledged control commands through the control virtqueue
> > > > > > > should be recorded, and we want to use shadow virtqueue to track dirty
> > > > > > > pages.
> > > > > > What you need to do is actually describe what do you expect the device
> > > > > > to do when it enters this suspend state. since you mention control
> > > > > > virtqueue then it seems that there needs to be a device type
> > > > > > specific text explaining the behaviour. If so this implies there
> > > > > > needs to be a list of device types that support suspend
> > > > > > until someone looks at each type and documents what it does.
> > > > > On a second thought, shadow vqs are hypervisor behaviors, maybe should not
> > > > > be
> > > > > described in this device spec.
> > > > > 
> > > > > Since SUSPEND is in device status, so for now I see every type of device
> > > > > implements
> > > > > device_status should support SUSPEND. This should be a general facility.


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