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

[Stefan Hajnoczi]

On Wed, Oct 27, 2021 at 01:22:28PM +0100, John Levon wrote:
> 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?

Yes. Actually I wasn't thinking of the vfio-user client but just about
QEMU device emulation code in general. What you suggested sounds like a
clean mapping from MemoryRegions to vfio-user.

Stefan

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