Re: [Qemu-devel] [RFC 0/2] virtio-vhost-user: add virtio-vhost-user device

[Jason Wang]

On 2018年01月25日 22:48, Michael S. Tsirkin wrote:
> On Thu, Jan 25, 2018 at 03:07:23PM +0100, Paolo Bonzini wrote:
> > On 23/01/2018 17:07, Michael S. Tsirkin wrote:
> > > > It's not clear to me how to do this. E.g need a way to report failure to VM2
> > > > or #PF?
> > > Why would there be a failure? qemu running vm1 would be responsible for
> > > preventing access to vm2's memory not mapped through an IOMMU.
> > > Basically munmap these.
> > Access to VM2's memory would use VM2's configured IOVAs for VM1's
> > requester id.  VM2's QEMU send device IOTLB messages to VM1's QEMU,
> > which would remap VM2's memory on the fly into VM1's BAR2.
> Right. Almost.

That would be extremely slow for dynamic mappings.

> One problem is that IOVA range is bigger than RAM range,
> so you will have trouble making arbitrary virtual addresses
> fit in a BAR.
>
> This is why I suggested a hybrid approach where
> translation happens within guest, qemu only does protection.
>
> Another problem with it is that IOMMU has page granularity
> while with hugetlbfs we might not be able to remap at that
> granularity.
>
> Not sure what to do about it - teach host to break
> up pages? Pass limitation to guest through virtio-iommu?

If we decide to go virtio IOMMU, maybe device can limit its IOVA range too.

Thanks

> Ideas?
>
> > It's not trivial to do it efficiently, but it's possible.  The important
> > thing is that, if VM2 has an IOMMU, QEMU must *not* connect to a
> > virtio-vhost-user device that lacks IOTLB support.  But that would be a
> > vhost-user bug, not a virtio-vhost-user bug---and that's the beauty of
> > Stefan's approach. :)
> >
> > Paolo