Re: [PATCH RFCv1 03/10] hw/arm/virt: Get the number of host-level SMMUv3 instances

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

On 7/9/24 19:11, Nicolin Chen wrote:
> On Tue, Jul 09, 2024 at 11:20:16AM +0200, Eric Auger wrote:
>> On 6/26/24 02:28, Nicolin Chen wrote:
>>> Nested SMMUv3 feature requires the support/presence of host-level SMMUv3
>>> instance(s). Add a helper to read the sysfs for the number of instances.
>>> Log them in a vms list using a new struct VirtNestedSmmu.
>>>
>>> This will be used by a following patch to assign a passthrough device to
>>> corresponding nested SMMUv3 instance.
>> Laterly the HostIOMMUDevice has been introduced to allow, among other
>> things, to pass information related to the physical IOMMU to the virtual
>> IOMMU.
>> I guess it would be well fitted to associate the viommu with its
>> underlying piommu.
> Wow, I missed that part -- backends/host_iommu_device. I will
> see how I can fit these well with that.
>
>> I don't think we have such kind of host introspection in machine type.
>> Generally in can happen in the very device or in libvirt.
> I think the biggest reason for having such an introspection in
> the virt code is because of hotplug, (though it's not properly
> implemented yet), as we don't know what new devices requiring
> for nested translation would be joining later. So somebody has
> to hold a full list.
>
> Would you mind elaborating how the "device" or "libvirt" can
> handle that?
If you know that on Grace you have 5 SMMU instances, can't you pre-build
a PCIe topology with 5 PXB and root ports at libvirt level.
Then when you hotplug your device you specify the corresponding slot
just as we do normally. But maybe I misunderstood the hotplug problematics.

Eric
>
> Thanks!
> Nicolin
>