Linux Virtualization - Apr 2016 - [PATCH RFC] fixup! virtio: convert to use DMA api

On Mon, 2016-04-18 at 16:12 +0300, Michael S. Tsirkin
wrote:
> I'm not sure I understand the issue.??The public API is not about how
> the driver works.??It doesn't say "don't use DMA API"
anywhere, does it?
> It's about telling device whether to obey the IOMMU and
> about discovering whether a device is in fact under the IOMMU.
Apologies, I was wrongly reading this as a kernel patch.

After a brief struggle with "telling device whether to obey the
IOMMU",
which is obviously completely impossible from the guest kernel, I
realise my mistake :)

So... on x86 how does this get reflected in the DMAR tables that the
guest BIOS presents to the guest kernel, so that the guest kernel
*knows* which devices are behind which IOMMU?

(And are you fixing the case of assigned PCI devices, which aren't
behind any IOMMU, at the same time as you answer that? :)

-- 
dwmw2


-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 5691 bytes
Desc: not available
URL:
<http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20160418/4933dd9a/attachment.bin>

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


-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 5691 bytes
Desc: not available
URL:
<http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20160418/d6b8e694/attachment-0001.bin>