[PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate_user

[Tian, Kevin]

> From: Jason Gunthorpe <jgg at nvidia.com>
> Sent: Friday, March 24, 2023 10:45 PM
> 
> > But still the main open for in-kernel short-path is what would be the
> > framework to move part of vIOMMU emulation into the kernel. If this
> > can be done cleanly then it's better than vhost-iommu which lacks
> > behind significantly regarding to advanced features. But if it cannot
> > be done cleanly leaving each vendor move random emulation logic
> > into the kernel then vhost-iommu sounds more friendly to the kernel
> >  though lots of work remains to fill the feature gap.
> 
> I assume there are reasonable ways to hook the kernel to kvm, vhost
> does it. I've never looked at it. At worst we need to factor some of
> the vhost code into some library to allow it.
> 
> We want a kernel thread to wakeup on a doorbell ring basically.
> 

kvm supports ioeventfd for the doorbell purpose.

Aside from that I'm not sure which part of vhost can be generalized
to be used by other vIOMMU. it's a in-memory ring structure plus
doorbell so it's easy to fit in the kernel.

But emulated vIOMMUs are typically MMIO-based ring structure
which requires 1) kvm provides a synchronous ioeventfd for MMIO
based head/tail emulation; 2) userspace vIOMMU shares its virtual
register page with the kernel which can then update virtual tail/head
registers w/o exiting to the userspace; 3) the kernel thread can
selectively exit to userspace for cmds which it cannot directly handle.

Those require a new framework to establish.