Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

[David Woodhouse]

On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> userspace registers a @port to an @eventfd, that is bound to a
>  @vcpu. This information is then used when the guest does an
> EVTCHNOP_send with a port registered with the kernel.

Why do I want this part?

> EVTCHNOP_send short-circuiting happens by marking the event as pending
> in the shared info and vcpu info pages and doing the upcall. For IPIs
> and interdomain event channels, we do the upcall on the assigned vcpu.

This part I understand, 'peeking' at the EVTCHNOP_send hypercall so
that we can short-circuit IPI delivery without it having to bounce
through userspace.

But why would I then want then short-circuit the short-circuit,
providing an eventfd for it to signal... so that I can then just
receive the event in userspace in a *different* form to the original
hypercall exit I would have got?

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