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Subject: Re: Enabling hypervisor agnosticism for VirtIO backends

On Wed, Aug 04, 2021 at 12:20:01PM -0700, Stefano Stabellini wrote:
> > Could we consider the kernel internally converting IOREQ messages from
> > the Xen hypervisor to eventfd events? Would this scale with other kernel
> > hypercall interfaces?
> > 
> > So any thoughts on what directions are worth experimenting with?
> One option we should consider is for each backend to connect to Xen via
> the IOREQ interface. We could generalize the IOREQ interface and make it
> hypervisor agnostic. The interface is really trivial and easy to add.
> The only Xen-specific part is the notification mechanism, which is an
> event channel. If we replaced the event channel with something else the
> interface would be generic. See:
> https://gitlab.com/xen-project/xen/-/blob/staging/xen/include/public/hvm/ioreq.h#L52

There have been experiments with something kind of similar in KVM
recently (see struct ioregionfd_cmd):

> There is also another problem. IOREQ is probably not be the only
> interface needed. Have a look at
> https://marc.info/?l=xen-devel&m=162373754705233&w=2. Don't we also need
> an interface for the backend to inject interrupts into the frontend? And
> if the backend requires dynamic memory mappings of frontend pages, then
> we would also need an interface to map/unmap domU pages.
> These interfaces are a lot more problematic than IOREQ: IOREQ is tiny
> and self-contained. It is easy to add anywhere. A new interface to
> inject interrupts or map pages is more difficult to manage because it
> would require changes scattered across the various emulators.

Something like ioreq is indeed necessary to implement arbitrary devices,
but if you are willing to restrict yourself to VIRTIO then other
interfaces are possible too because the VIRTIO device model is different
from the general purpose x86 PIO/MMIO that Xen's ioreq seems to support.


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