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

[John Levon]

On Wed, Oct 27, 2021 at 11:15:35AM +0100, Stefan Hajnoczi wrote:

> > > 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?
> 
> The ioregionfd doesn't need to cover an entire BAR. QEMU's MemoryRegions
> form a hierarchy, so it's possible to sub-divide the BAR into several
> MemoryRegions.

I think you're saying that when vfio-user client in qemu calls
VFIO_USER_DEVICE_GET_REGION_IO_FDS, it would create a sub-MR corresponding to
each one, before asking KVM to configure them?

thanks
john