Re: MMIO/PIO dispatch file descriptors (ioregionfd) design discussion

[John Levon]

On Mon, Oct 25, 2021 at 08:21:22AM -0700, Elena wrote:

> > I'm curious what approach you want to propose for QEMU integration. A
> > while back I thought about the QEMU API. It's possible to implement it
> > along the lines of the memory_region_add_eventfd() API where each
> > ioregionfd is explicitly added by device emulation code. An advantage of
> > this approach is that a MemoryRegion can have multiple ioregionfds, but
> > I'm not sure if that is a useful feature.
> >
> 
> This is the approach that is currently in the works. Agree, I dont see
> much of the application here at this point to have multiple ioregions
> per MemoryRegion.
> I added Memory API/eventfd approach to the vfio-user as well to try
> things out.
> 
> > An alternative is to cover the entire MemoryRegion with one ioregionfd.
> > That way the device emulation code can use ioregionfd without much fuss
> > since there is a 1:1 mapping between MemoryRegions, which are already
> > there in existing devices. There is no need to think deeply about which
> > ioregionfds to create for a device.
> >
> > A new API called memory_region_set_aio_context(MemoryRegion *mr,
> > AioContext *ctx) would cause ioregionfd (or a userspace fallback for
> > non-KVM cases) to execute the MemoryRegion->read/write() accessors from
> > the given AioContext. The details of ioregionfd are hidden behind the
> > memory_region_set_aio_context() API, so the device emulation code
> > doesn't need to know the capabilities of ioregionfd.
> 
> > 
> > The second approach seems promising if we want more devices to use
> > ioregionfd inside QEMU because it requires less ioregionfd-specific
> > code.
> > 
> I like this approach as well.
> As you have mentioned, the device emulation code with first approach
> does have to how to handle the region accesses. The second approach will
> make things more transparent. Let me see how can I modify what there is
> there now and may ask further questions.

Sorry I'm a bit late to this discussion, I'm not clear on the above WRT
vfio-user. If an ioregionfd has to cover a whole BAR0 (?), how would this
interact with partly-mmap()able regions like we do with SPDK/vfio-user/NVMe?

thanks
john