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

[Jason Wang <jasowang@redhat.com>]

On 2021/2/9 上午2:26, Peter Xu wrote:
> 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.


Note that on top of device-IOTLB, device may choose to implement an 
IOMMU which support #PF. In this case, dev-iotlb semantic is not a must. 
(Or it can co-operate with things like ATS if driver wants)

Virtio will probably provide this feature in the future.

Thanks


>
> 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,
>