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

[Jason Wang]

On 2021/2/9 上午2:37, Peter Xu wrote:
> On Mon, Feb 08, 2021 at 11:21:23AM +0800, Jason Wang wrote:
>
> [...]
>
> > > I'm not sure I remember it right, but we seem to have similar discussion
> > > previously on "what if the user didn't specify ats=on" - I think at that time
> > > the conclusion was that we ignore the failure since that's not a valid
> > > configuration for qemu.
> >
> > Yes, but I think I was wrong at that time.
> I can't say you're wrong - I actually still agree with you that at least
> there's a priority of things we'd do, and this one is not extremely important
> if that's not a major use case (say, if you will 100% always suggest an user to
> use ats=on for a viommu enabled vhost).


Right, but it depends on e.g how libvirt use that. As far as I know, 
they do enable ATS. But it would still an issue if libvirt want to 
support vIOMMUs other than intel.


> > > The other issue I'm worried is (I think I mentioned it somewhere, but just to
> > > double confirm): I'd like to make sure SMMU and virtio-iommu are the only IOMMU
> > > platform that will use vhost.
> >
> > For upstream, it won't be easy :)
> Sorry I definitely didn't make myself clear... :)
>
> To be explicit, does ppc use vhost kernel too?


I think the answer is yes.


>   Since I know at least ppc has
> its own translation unit and its iommu notifier in qemu, so I'm unsure whether
> the same patch would break ppc too, because vhost could also ignore all UNMAP
> sent by the ppc vIOMMU.


If this is true, we probably need to fix that.


> > >     Otherwise IIUC we need to fix those vIOMMUs too.
> >
> > Right, last time I check AMD IOMMU emulation, it simply trigger device IOTLB
> > invalidation during IOTLB invalidation which looks wrong.
> I did quickly grep IOMMU_NOTIFIER_UNMAP in amd_iommu.c and saw nothing. It
> seems amd iommu is not ready for any kind of IOMMU notifiers yet.
>
> Thanks,


Right.

Thanks


>