[nellydpa]

You are right, some instructions are not suitable for userspace due to their performance implications and have to stay in the kernel. We identified a small set of them, for example, some parts of IOAPIC support have to stay put.

[bonzini]

LAPIC I think? But those get their own special vmexit code so they do not need emulation (on Ivy Bridge or newer Xeons).

IOAPIC is legacy and replaced by MSI. I am surprised you don't use ioeventfd though!

[jsolson]

> I am surprised you don't use ioeventfd though!

We do in some cases, for both networking and storage. Since our devices are (mostly) VIRTIO (of pre-1.0 vintage), we're using it for OUTs into BAR0 (which again of course get their own VMEXIT and don't require emulation).

By and large we try to elide the exits entirely if we can, naturally, although in today's GCE production environment serialized request/response type workloads will see exits on every packet. Streaming workloads fare better, as we do make use of EVENT_IDX and aggressively trying to find more work before advancing the used.avail_idx field.