On Mon, 2016-04-18 at 17:23 +0300, Michael S. Tsirkin wrote: > > This patch doesn't change DMAR tables, it creates a way for virtio > device to tell guest "I obey what DMAR tables tell you, you can stop > doing hacks". > > And as PPC guys seem adamant that platform tools there are no good for > that purpose, there's another bit that says "ignore what platform tells > you, I'm not a real device - I'm part of hypervisor and I bypass the > IOMMU". ... +/* Request IOMMU passthrough (if available) + * Without VIRTIO_F_IOMMU_PLATFORM: bypass the IOMMU even if enabled. + * With VIRTIO_F_IOMMU_PLATFORM: suggest disabling IOMMU. + */ +#define VIRTIO_F_IOMMU_PASSTHROUGH 33 + +/* Do not bypass the IOMMU (if configured) */ +#define VIRTIO_F_IOMMU_PLATFORM 34 OK... let's see if I can reconcile those descriptions coherently. Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that its own operating system's IOMMU code is expected to be broken, and that the virtio driver should eschew the DMA API? And that the guest OS cannot further assign the affected device to any of *its* nested guests? Not that the broken IOMMU code in said guest OS will know the latter, of course. With VIRTIO_F_IOMMU_PLATFORM set, VIRTIO_F_IOMMU_PASSTHROUGH is just a *hint*, suggesting that the guest OS should *request* a passthrough mapping from the IOMMU? Via a driver←→IOMMU API which doesn't yet exist in Linux, since we only have 'iommu=pt' on the command line for that? And having *neither* of those bits sets is the status quo, which means that your OS code might well be broken and need you to eschew the DMA API, but maybe not. -- dwmw2
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