On 12/21/09 11:40 AM, Avi Kivity wrote: > On 12/21/2009 06:37 PM, Anthony Liguori wrote: >> Since virtio-pci supports MSI-X, there should be no IO exits on >> host->guest notification other than EOI in the virtual APIC. This is >> a light weight exit today and will likely disappear entirely with >> newer hardware. > > I'm working on disappearing EOI exits on older hardware as well. Same > idea as the old TPR patching, without most of the magic. > While I applaud any engineering effort that results in more optimal execution, if you are talking about what we have discussed in the past its not quite in the same league as my proposal. You are talking about the ability to optimize the final EOI if there are no pending interrupts remaining, right? The problem with this approach is it addresses the wrong side of the curve: That is, it optimizes the code as its about to go io-idle. You still have to take an extra exit for each injection during the heat of battle, which is when you actually need it most. To that front, what I have done is re-used the lockless shared-memory concept for even "interrupt injection". Lockless shared-memory rings have the property that both producer and consumer can simultaneously manipulate the ring. So what we do in alacrityvm is deliver shm-signals (shm-signal == "interrupt" in vbus) over a ring so that the host can inject a signal to a running vcpu and a vcpu can complete an ack/re-inject cycle directly from vcpu context. Therefore, we only need a physical IDT injection when the vcpu is io-idle transitioning to io-busy, and remain completely in parallel guest/host context until we go idle again. That said, your suggestion would play nicely with the above mentioned scheme, so I look forward to seeing it in the tree. Feel free to send me patches for testing. Kind Regards, -Greg
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