Re: [PATCH 0/5] Convert virtio-iommu to domain_alloc_paging()

Jean-Philippe Brucker
2025-02-21T11:41:43Z

On Wed, Feb 12, 2025 at 08:50:07AM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 11, 2025 at 04:41:51PM -0800, Jacob Pan wrote:
>
> > I see the same on arm64 with v9.0, assigned an ixgbe nic via VFIO.
>
> Huh, I figured it worked on ARM..

Crosvm would be a more useful test because it doesn't implement any BYPASS
feature, but I don't have it set up at the moment. It's not a problem on
QEMU since it supports the BYPASS feature and doesn't need identity-mapped
domains.

>
> There is no requirement that an iommu driver implement identity, this
> is all an optimization. If it doesn't work in alot of cases can we
> just remove it? It would simplfy a lot..

No objection. Identity domains are useful if the use-case of virtio-iommu
is assigning devices to guest userspace, in which case you'd want to
bypass the IOMMU for the devices that are still owned by the kernel. But
VMMs have at least two alternatives to identity-mapping (BYPASS feature
and tailored topology).

Thanks,
Jean