Re: [Qemu-devel] [PATCH RFC 00/11] qemu: towards virtio-1 host support

[Jan Kiszka]

On 2014-10-22 22:34, Benjamin Herrenschmidt wrote:
> On Wed, 2014-10-22 at 16:17 +0200, Jan Kiszka wrote:
>> I thought about this again, and I'm not sure anymore if we can use
>> ACPI
>> to "black-list" the incompatible virtio devices. Reason: hotplug. To
>> my
>> understanding, the ACPI DRHD tables won't change during runtime when a
>> device shows up or disappears. We would have to isolate virtio devices
>> from the rest of the system by using separate buses for it (and avoid
>> listing those in any DRHD table) and enforce that they only get
>> plugged
>> into those buses. I suppose that is not desirable.
>>
>> Maybe it's better to fix virtio /wrt IOMMUs.
> 
> I always go back to my initial proposal which is to define that current
> virtio always bypass any iommu (which is what it does really) and have
> it expose via a new capability if that isn't the case. That means fixing
> that Xen thingy to allow qemu to know what to expose I assume but that
> seems to be the less bad approach.

Just one thing to consider: feature negotiation happens after guest
startup. If we run a virtio device under IOMMU control, what will we
have to do when the guest says it does not support such devices? Simply
reject operation?

Jan

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