On Fri, 2024-07-26 at 09:04 -0400, Michael S. Tsirkin wrote: > 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. For clarity, I think I've ditched the idea of a poor-man's negotiation through invoking an ACPI method to enable the timekeeping info. I think we're better off waiting for virtio, to enable that kind of thing. The guest gets what the hypervisor is prepared to offer "for free". That isn't a one-way door; we *can* add an optional ACPI method later if we really want to. But I'm generally OK with "use virtio if you want that".
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