Re: [RFC] /dev/ioasid uAPI proposal

[Alex Williamson]

On Tue, 1 Jun 2021 07:01:57 +0000
"Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> I summarized five opens here, about:
> 
> 1)  Finalizing the name to replace /dev/ioasid;
> 2)  Whether one device is allowed to bind to multiple IOASID fd's;
> 3)  Carry device information in invalidation/fault reporting uAPI;
> 4)  What should/could be specified when allocating an IOASID;
> 5)  The protocol between vfio group and kvm;
> 
...
> 
> For 5), I'd expect Alex to chime in. Per my understanding looks the
> original purpose of this protocol is not about I/O address space. It's
> for KVM to know whether any device is assigned to this VM and then
> do something special (e.g. posted interrupt, EPT cache attribute, etc.).

Right, the original use case was for KVM to determine whether it needs
to emulate invlpg, so it needs to be aware when an assigned device is
present and be able to test if DMA for that device is cache coherent.
The user, QEMU, creates a KVM "pseudo" device representing the vfio
group, providing the file descriptor of that group to show ownership.
The ugly symbol_get code is to avoid hard module dependencies, ie. the
kvm module should not pull in or require the vfio module, but vfio will
be present if attempting to register this device.

With kvmgt, the interface also became a way to register the kvm pointer
with vfio for the translation mentioned elsewhere in this thread.

The PPC/SPAPR support allows KVM to associate a vfio group to an IOMMU
page table so that it can handle iotlb programming from pre-registered
memory without trapping out to userspace.

> Because KVM deduces some policy based on the fact of assigned device, 
> it needs to hold a reference to related vfio group. this part is irrelevant
> to this RFC. 

All of these use cases are related to the IOMMU, whether DMA is
coherent, translating device IOVA to GPA, and an acceleration path to
emulate IOMMU programming in kernel... they seem pretty relevant.

> But ARM's VMID usage is related to I/O address space thus needs some
> consideration. Another strange thing is about PPC. Looks it also leverages
> this protocol to do iommu group attach: kvm_spapr_tce_attach_iommu_
> group. I don't know why it's done through KVM instead of VFIO uAPI in
> the first place.

AIUI, IOMMU programming on PPC is done through hypercalls, so KVM needs
to know how to handle those for in-kernel acceleration.  Thanks,

Alex