On Tue, Oct 26, 2021 at 08:01:39PM +0100, John Levon wrote: > 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? 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. This means it's still possible to have mmap() sub-regions or even ioeventfds sprinkled in between. Stefan
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