On Mon, 2020-11-30 at 15:08 +0000, Joao Martins wrote: > On 11/30/20 12:55 PM, David Woodhouse wrote: > > On Mon, 2020-11-30 at 12:17 +0000, Joao Martins wrote: > > > On 11/30/20 9:41 AM, David Woodhouse wrote: > > > > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote: > > > One thing I didn't quite do at the time, is the whitelisting of > > > unregistered > > > ports to userspace. Right now, it's a blacklist i.e. if it's not handled > > > in > > > the kernel (IPIs, timer vIRQ, etc) it goes back to userspace. When the > > > only > > > ones which go to userspace should be explicitly requested as such > > > and otherwise return -ENOENT in the hypercall. > > > > Hm, why would -ENOENT be a fast path which needs to be handled in the > > kernel? > > > > It's not that it's a fast path. > > Like sending an event channel to an unbound vector, now becomes an possible > vector to > worry about in userspace VMM e.g. should that port lookup logic be fragile. > > So it's more along the lines of Nack-ing the invalid port earlier to rather go > to go userspace to invalidate it, provided we do the lookup anyway in the > kernel.
If the port lookup logic is fragile, I *want* it in the sandboxed userspace VMM and not in the kernel :) And unless we're going to do *all* of the EVTCHNOP_bind*, EVTCHN_close, etc. handling in the kernel, doesn't userspace have to have all that logic for managing the port space anyway? I think it's better to let userspace own it outright, and use the kernel bypass purely for the fast paths. The VMM can even implement IPI/VIRQ support in userspace, then use the kernel bypass if/when it's available. > > > Perhaps eventfd could be a way to express this? Like if you register > > > without an eventfd it's offloaded, otherwise it's assigned to userspace, > > > or if neither it's then returned an error without bothering the VMM. > > > > I much prefer the simple model where the *only* event channels that the > > kernel knows about are the ones it's expected to handle. > > > > For any others, the bypass doesn't kick in, and userspace gets the > > KVM_EXIT_HYPERCALL exit. > > > > /me nods > > I should comment on your other patch but: if we're going to make it generic > for > the userspace hypercall handling, might as well move hyper-v there too. In > this series, > I added KVM_EXIT_XEN, much like it exists KVM_EXIT_HYPERV -- but with a > generic version > I wonder if a capability could gate KVM_EXIT_HYPERCALL to handle both guest > types, while > disabling KVM_EXIT_HYPERV. But this is probably subject of its own separate > patch :) There's a limit to how much consolidation we can do because the ABI is different; the args are in different registers. I do suspect Hyper-V should have marshalled its arguments into the existing kvm_run->arch.hypercall and used KVM_EXIT_HYPERCALL but I don't think it makes sense to change it now since it's a user-facing ABI. I don't want to follow its lead by inventing *another* gratuitous exit type for Xen though.
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