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

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

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. 

> 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.

> > 
> > > > > But I still believe we are here try our best to work out an industrial spec
> > > > > with better quality, to serve broad interest. This is not competition
> > > > > between companies,
> > > > > and the spec is not a FIFO, not like a early bird can catch all the worm.


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