Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api

[David Woodhouse <dwmw2@infradead.org>]

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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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