Using the KVM API [LWN.net]

Using the KVM API

Posted Sep 30, 2015 17:53 UTC (Wed) by josh (subscriber, #17465)
In reply to: Using the KVM API by pbonzini
Parent article: Using the KVM API

> For ioeventfd you have to add the latency of waking up the I/O thread if it's sleeping (but if the fd is really busy, e.g. running fio in the guest, it won't have time to go to sleep).

How much *minimum* latency comes from the vmexit, and how much gets added by the path from the in-kernel vmexit handling and whatever mechanism it uses to contact the I/O thread? If much of it comes from the latter, perhaps we could find a way to accelerate that via another (latency-optimized) interface.

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Using the KVM API

Posted Oct 1, 2015 7:17 UTC (Thu) by pbonzini (subscriber, #60935) [Link] (2 responses)

It's about 50-50.

Using the KVM API

Posted Oct 1, 2015 15:50 UTC (Thu) by josh (subscriber, #17465) [Link] (1 responses)

Ouch. That really limits how much faster it can get, then, short of implementing the backend in the kernel.

Using the KVM API

Posted Oct 1, 2015 16:02 UTC (Thu) by pbonzini (subscriber, #60935) [Link]

Even if you implement in the kernel, it's hard to bypass all the stuff that KVM does at vmexit. The processor doesn't restore the whole host state at vmexit time, but KVM must restore at least whatever is needed to run Linux (e.g. the %fs base to access per_cpu variables).