Re: [PATCH vfio 10/11] vfio/virtio: Expose admin commands over virtio device

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On Mon, Oct 16, 2023 at 04:33:10PM +0800, Zhu, Lingshan wrote:
> 
> 
> On 10/13/2023 9:50 PM, Michael S. Tsirkin wrote:
> > On Fri, Oct 13, 2023 at 06:28:34PM +0800, Zhu, Lingshan wrote:
> > > 
> > > On 10/12/2023 9:27 PM, Jason Gunthorpe wrote:
> > > 
> > >      On Thu, Oct 12, 2023 at 06:29:47PM +0800, Zhu, Lingshan wrote:
> > > 
> > > 
> > >          sorry for the late reply, we have discussed this for weeks in virtio mailing
> > >          list. I have proposed a live migration solution which is a config space solution.
> > > 
> > >      I'm sorry that can't be a serious proposal - config space can't do
> > >      DMA, it is not suitable.
> > > 
> > > config space only controls the live migration process and config the related
> > > facilities.
> > > We don't use config space to transfer data.
> > > 
> > > The new added registers work like queue_enable or features.
> > > 
> > > For example, we use DMA to report dirty pages and MMIO to fetch the dirty data.
> > > 
> > > I remember in another thread you said:"you can't use DMA for any migration
> > > flows"
> > > 
> > > And I agree to that statement, so we use config space registers to control the
> > > flow.
> > > 
> > > Thanks,
> > > Zhu Lingshan
> > > 
> > > 
> > >      Jason
> > > 
> > If you are using dma then I don't see what's wrong with admin vq.
> > dma is all it does.
> dma != admin vq,

Well they share the same issue that they don't work for nesting
because DMA can not be intercepted.

> and I think we have discussed many details in pros and cons
> in admin vq live migration proposal in virtio-comment.
> I am not sure we should span the discussions here, repeat them over again.
> 
> Thanks
> > 

Yea let's not.

-- 
MST

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