On Fri, Jul 26, 2024 at 02:00:25PM +0100, David Woodhouse wrote: > On Fri, 2024-07-26 at 08:52 -0400, Michael S. Tsirkin wrote: > > On Fri, Jul 26, 2024 at 09:35:51AM +0100, David Woodhouse wrote: > > > But for this use case, we only need a memory region that the hypervisor > > > can update. We don't need any of that complexity of gratuitously > > > interrupting all the vCPUs just to ensure that none of them can be > > > running userspace while one of them does an update for itself, > > > potentially translating from one ABI to another. The hypervisor can > > > just update the user-visible memory in place. > > > > Looks like then your userspace is hypervisor specific, and that's a > > problem because it's a one way street - there is no way for hypervisor > > to know what does userspace need, so no way for hypervisor to know which > > information to provide. No real way to fix bugs. > > It's not hypervisor specific, but you're right that as it stands there > is no negotiation of what userspace wants. So the hypervisor provides > what it feels it can provide without significant overhead (which may or > may not include the precise timekeeping, as discussed, but should > always include the disruption signal which is the most important > thing). > > The guest *does* know what the hypervisor provides. And when we get to > do this in virtio, we get all the goodness of negotiation as well. The > existence of the simple ACPI model doesn't hurt that at all. Maybe it doesn't, at that. E.g. virtio does a copy, acpi doesn't? I'll ponder compatibility over the weekend. -- MST
