On Fri, Aug 05, 2022 at 03:57:08PM -0700, Linus Torvalds wrote: > Why is "IOMMU support" called "VIRTIO_F_ACCESS_PLATFORM"? Because, as far as the virtio spec and virtio "guest" implementation is concerned it is not about IOMMU support at all. It is about treating virtio DMA as real DMA by the platform, which lets the platform let whatever method of DMA mapping it needs to the virtio device. This is needed to make sure harware virtio device are treated like actual hardware and not like a magic thing bypassing the normal PCIe rules. Using an IOMMU if one is present for bus is just one thing, others are using offets of DMAs that are very common on non-x86 platforms, or doing the horrible cache flushing needed on devices where PCIe is not cache coherent. It really is vhost that seems to abuse it so that if the guest claims it can handle VIRTIO_F_ACCESS_PLATFORM (which every modern guest should) it enables magic behavior, which I don't think is what the virtio spec intended.
