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

From: Jason Wang
Subject: Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support
Date: Tue, 9 Feb 2021 11:12:52 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

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.




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