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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list
On Fri, 2016-06-24 at 14:49 +0100, George Dunlap wrote: > On 24/06/16 14:42, Wu, Feng wrote: > > Here is my understanding, if the guest has nothing to do, it will > > call HLT, and Xen hypervisor will call vcpu_block(), if we don't > > block the vCPU and return to guest, guest will continue to run > > HLT if it still has nothing to do. > As it happens, most operating systems will handle this situation just > fine; but I'm not sure this is something we want to rely on. > Exactly! I indeed don't think it would be wise to rely on that. But more than that, I don't think we want to waste pCPU cycles considering runnable, and hence potentially scheduling, vCPUs that have HTL-ed! That may be acceptable in emulators like Bochs or "plain" Qemu... Xen is an hypervisor, it does virtualization, and this is one of the points of virtualization, IMO: when a virtual CPU has nothing to do, use the physical CPU to execute _another_ virtual CPU. And even if that would be just for the sake of handling a corner case, I still think that: - we really want to try as hard as possible to avoid it, - if we really can't, we want a big comment in the code for: * explaining what actually happens, * proving that it will be something limited in time (and preferably rather short), * providing an explanations of why we could not do better. Regards, Dario -- <<This happens because I choose it to happen!>> (Raistlin Majere) ----------------------------------------------------------------- Dario Faggioli, Ph.D, http://about.me/dario.faggioli Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK) Attachment:
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