Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support

[Peter Xu <peterx@redhat.com>]

Kevin,

On Mon, Feb 08, 2021 at 07:03:08AM +0000, Tian, Kevin wrote:
> It really depends on the definition of dev-iotlb in this context. To me the
> fact that virtio-iommu needs to notify the kernel for updating split cache
> is already sort of dev-iotlb semantics, regardless of whether it's delivered 
> through a iotlb message or dev-iotlb message in a specific implementation. 😊

Yeah maybe it turns out that we'll just need to implement dev-iotlb for
virtio-iommu.

I am completely fine with that and I'm never against it. :) I was throwing out
a pure question only, because I don't know the answer.

My question was majorly based on the fact that dev-iotlb and iotlb messages
really look the same; it's not obvious then whether it would always matter a
lot when in a full emulation environment.

One example is current vhost - vhost previously would work without dev-iotlb
(ats=on) because trapping UNMAP would work too for vhost to work.  It's also
simply because at least for VT-d the driver needs to send both one dev-iotlb
and one (probably same) iotlb message for a single page invalidation.  The
dev-iotlb won't help a lot in full emulation here but instead it slows thing
down a little bit (QEMU has full knowledge as long as it receives either of the
message).

Thanks,

-- 
Peter Xu