Re: [PATCH v5 2/3] virtio_pci: Use the DMA API for virtqueues when possible

On Wed, Sep 24, 2014 at 3:38 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Wed, 2014-09-24 at 15:15 -0700, Andy Lutomirski wrote:

quoted

On Wed, Sep 24, 2014 at 3:04 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

quoted

On Wed, 2014-09-24 at 14:59 -0700, Andy Lutomirski wrote:

quoted

Scratch that idea, then.

The best that I can currently come up with is to say that pre-1.0
devices on PPC bypass the IOMMU and that 1.0 devices on PPC and all
devices on all other architectures do not bypass the IOMMU.

Well, the thing is, we *want* them to bypass the IOMMU for performance
reasons in the long run. Today we have no ways to tell our guests that
a PCI bus doesn't have an IOMMU, they always do !

Alternatively, the IOMMU could be extended to allow an efficient
identity map of the entire physical address space.

We have that option, but I'm a bit reluctant to rely on it, it has its
issues, but it's something we can look into.

quoted

quoted

Also qemu can mix and match devices on a given PCI bus so making this a
bus property wouldn't work either.

quoted

I agree that this is far from ideal.  Any ideas?  Is there any other
channel that QEMU can use to signal to a PPC guest that a specific PCI
virtio device isn't really behind an IOMMU?

Any reason why this can't be a virtio capability ?

For Xen.

And ? I don't see the problem... When running Xen, you don't put the
capability in. They are negociated... Why can't qemu set the capability
accordingly based on whether it's using KVM or Xen ? It's not like we
care about backward compat of ppc on Xen anyway...

QEMU doesn't know that Xen is running.  I can do qemu-system-x86_64
-kernel /path/to/xen (virtme-run --xen does this) or
qemu-system-x86_64 -hda disk_with_xen.img and Xen will run.

 .../...

quoted

a) IOMMU should be used.
b) Device is trusted but the IOMMU still works.  The guest may want to
set up an identity map.
c) (PPC only) Device bypasses the IOMMU.  There's no security issue
here: a malicious device wouldn't be able to bypass the IOMMU, so it
would be unable to do DMA at all.

I wouldn't make it "ppc only" at this point but yes. Also keep in mind
that we do have ppc cases where we will want to use the iommu (in which
case the capability would be enforced by the other half).

The only reason I say "PPC only" is because I don't think that other
architectures would want to choose c.

We could plausibly use paravirt tricks for this, but that gets messy,
since e.g. the KVM kernel code might not even know what QEMU wants to
do.

--Andy