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It could be anything that can help > > > us distinguish between a non-grants-capable virtio backend and a > > > grants-capable virtio backend, such as: > > > - a string on xenstore > > > - a xen param > > > - a special PCI configuration register value > > > - something in the ACPI tables > > > - the QEMU machine type > > > > > > Yes, I remember there was a discussion regarding that. The point is to > > choose a solution to be functional for both PV and HVM *and* to be able > > to support a hotplug. IIRC, the xenstore could be a possible candidate. > > xenstore would be among the easiest to make work. The only downside is > the dependency on xenstore which otherwise virtio+grants doesn't have. I would avoid introducing a dependency on xenstore, if nothing else we know it's a performance bottleneck. We would also need to map the virtio device topology into xenstore, so that we can pass different options for each device. > Vikram is working on virtio with grants support in QEMU as we speak. > Maybe we could find a way to add a flag in QEMU so that we can detect at > runtime if a given virtio device support grants or not. Isn't there a way for the device to expose capabilities already? For example how does a virtio-blk backend expose support for indirect descriptors? Thanks, Roger.