> On Feb 28, 2024, at 11:38 AM, Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
>
> On Wed, 28 Feb 2024 10:29:32 -0800
> Peter Delevoryas <peter@xxxxxxx> wrote:
>
>> Hey guys,
>>
>> I’m having a little trouble reading between the lines on various
>> docs, mailing list threads, KVM presentations, github forks, etc, so
>> I figured I’d just ask:
>>
>> What is the status of IOMMU virtualization, like in the case where I
>> want a VM guest to have a virtual IOMMU?
>
> It works fine for simply nested assignment scenarios, ie. guest
> userspace drivers or nested VMs.
>
>> I found this great presentation from KVM Forum 2021: [1]
>>
>> 1. I’m using -device intel-iommu right now. This has performance
>> implications and large DMA transfers hit the vfio_iommu_type1
>> dma_entry_limit on the host because of how the mappings are made.
>
> Hugepages for the guest and mappings within the guest should help both
> the mapping performance and DMA entry limit. In general the type1 vfio
> IOMMU backend is not optimized for dynamic mapping, so performance-wise
> your best bet is still to design the userspace driver for static DMA
> buffers.
Yep, huge pages definitely help, will probably switch to allocating them at boot for better guarantees.
>
>> 2. -device virtio-iommu is an improvement, but it doesn’t seem
>> compatible with -device vfio-pci? I was only able to test this with
>> cloud-hypervisor, and it has a better vfio mapping pattern (avoids
>> hitting dma_entry_limit).
>
> AFAIK it's just growing pains, it should work but it's working through
> bugs.
Oh really?? Ok: I might even be configuring things incorrectly, or
Maybe I need to upgrade from QEMU 7.1 to 8. I was relying on whatever
libvirt does by default, which seems to just be:
-device virtio-iommu -device vfio-pci,host=<bdf>
But maybe I need some other options?
>
>> 3. -object iommufd [2] I haven’t tried this quite yet, planning to:
>> if it’s using iommufd, and I have all the right kernel features in
>> the guest and host, I assume it’s implementing the passthrough mode
>> that AMD has described in their talk? Because I imagine that would be
>> the best solution for me, I’m just having trouble understanding if
>> it’s actually related or orthogonal.
>
> For now iommufd provides a similar DMA mapping interface to type1, but
> it does remove the DMA entry limit and improves locked page accounting.
>
> To really see a performance improvement relative to dynamic mappings,
> you'll need nesting support in the IOMMU, which is under active
> development. From this aspect you will want iommufd since similar
> features will not be provided by type1. Thanks,
I see, thanks! That’s great to hear.
>
> Alex
>
