Re: [Qemu-devel] [RFC v4 00/16] VIRTIO-IOMMU device

[Auger Eric <eric.auger@redhat.com>]

Hi Peter,

On 12/10/2017 11:54, Peter Maydell wrote:
> On 11 October 2017 at 17:08, Auger Eric <eric.auger@redhat.com> wrote:
>> Hi Peter,
>>
>> On 11/10/2017 16:56, Peter Maydell wrote:
>>> On 19 September 2017 at 08:46, Eric Auger <eric.auger@redhat.com> wrote:
>>>> This series implements the virtio-iommu device.
>>>>
>>>> This v4 is an upgrade to v0.4 spec [1] and applies on QEMU v2.10.0.
>>>> - probe request support although no reserved region is returned at
>>>>   the moment
>>>> - unmap semantics less strict, as specified in v0.4
>>>> - device registration, attach/detach revisited
>>>> - split into smaller patches to ease review
>>>> - propose a way to inform the IOMMU mr about the page_size_mask
>>>>   of underlying HW IOMMU, if any
>>>> - remove warning associated with the translation of the MSI doorbell
>>>>
>>>> The device gets instantiated using the "-device virtio-iommu-device"
>>>> option. It currently works with ARM virt machine only, as the machine
>>>> must handle the dt binding between the virtio-mmio "iommu" node and
>>>> the PCI host bridge node.
>>>
>>> Could this work on x86, or is it inherently arm-only?
>>
>> Yes this is the goal. At the moment the ACPI probing is not yet properly
>> specified but a Q35 prototype was developed in the Red Hat Virt team.
>> This will be presented at the KVM forum.
> 
> Since I have very little familiarity with virtio or iommu code,
> I'd be much happier if this was reviewed as a generic virtio-iommu
> by the x86/virtio devs and then the arm specific parts done second...

Understood. I was rather expecting you to review the smmuv3 emulation
code which you did, in a comprehensive manner ;-), and many thanks for that.

Note sure this is time yet to get this RFC reviewed as
- the v0.4 virtio-iommu driver it relies on was not officially submitted,
- the virtio-iommu specification review has not really been reviewed,
- the ACPI probing method has not been discussed yet.

Jean-Philippe, please correct me if I am wrong.

So to me, this is pure RFC at the moment.
> 
> I'm also not clear on what we're expecting the recommended or normal
> way to do device passthrough is going to be -- this virtio-mmio,
> or presenting the guest with an SMMUv3 interface? Do we really
> need to implement both ?

I think the KVM forum is the right place to sync as both approaches will
be presented and some pros/cons + performance figures will be given.

As we talk about choosing, there is one alternative that was suggested
on the ML by Alex & Michael but never really get considered yet and
maybe should be: using intel iommu emulation code for ARM. I aknowledge
this deserves a thorough impact study on kernel and FW side but I would
be happy to get your opinion about the QEMU side. Would you have a by-
principle rejection of this idea to instantiate such an Intel device in
mach virt or would it be something you would be ready to consider?

Thanks

Eric


> 
> thanks
> -- PMM
>