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Subject: Re: [virtio-dev] Re: [PATCH v1 6/6] vhost-user: add VFIO based accelerators support

On 2018年01月26日 07:59, Michael S. Tsirkin wrote:
The virtual IOMMU isn't supported by the accelerators for now.
Because vhost-user currently lacks of an efficient way to share
the IOMMU table in VM to vhost backend. That's why the software
implementation of virtual IOMMU support in vhost-user backend
can't support dynamic mapping well.
What exactly is meant by that? vIOMMU seems to work for people,
it's not that fast if you change mappings all the time,
but e.g. dpdk within guest doesn't.

Yes, software implementation support dynamic mapping for sure. I think the point is, current vhost-user backend can not program hardware IOMMU. So it can not let hardware accelerator to cowork with software vIOMMU. I think that's another call to implement the offloaded path inside qemu which has complete support for vIOMMU co-operated VFIO.


Once this problem is solved
in vhost-user, virtual IOMMU can be supported by accelerators
too, and the IOMMU feature bit checking in this patch can be
Given it works with software backends right now, I suspect
this will be up to you guys to address.

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