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

[Stefan Hajnoczi]

On Mon, Oct 25, 2021 at 08:21:22AM -0700, Elena wrote:
> On Mon, Oct 25, 2021 at 01:42:56PM +0100, Stefan Hajnoczi wrote:
> > On Mon, Oct 11, 2021 at 10:34:29PM -0700, elena wrote:
> > > On Wed, Nov 25, 2020 at 12:44:07PM -0800, Elena Afanasova wrote:
> > > > Hello,
> > > >
> > > 
> > > Hi
> > > 
> > > Sorry for top-posting, just wanted to provide a quik update.
> > > We are currently working on the support for ioregionfd in Qemu and will
> > > be posting the patches soon. Plus the KVM patches will be posted based
> > > of the RFC v3 with some fixes if there are no objections from Elena's side
> > > who originally posted KVM RFC patchset.
> > 
> > Hi Elena,
> 
> Hello Stefan.
> 
> > 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.

Thanks, I look forward to patches you are working on!

Stefan

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