On 04.06.14 16:39, Gabriel L. Somlo wrote:
Paolo, I noticed the monitor=mwait=nop patch is making its way upstream, so thanks ! I'm still interested in following up with something that would enable this behavior only conditionally (e.g. following an ioctl call from userspace to enable it only for the (set of) vcpu(s) belonging to one guest VM at a time), which should then also include advertising the feature in CPUID. I grep-ed through the kvm sources for KVM_CAP for some inspiration, and it looks more like KVM_CAP_* is a way to tell userspace what the kernel supports, but nothing I saw showed me an example of a "tunable" feature that userspace may ask to be turned on or off (e.g per-vcpu). Is there something like that I could use as an example ?
Sure, we use it all over the place on PPC :).
Obviously, if you really like the current behavior better you can always reject whatever patch I'll come up with, but I'd like to at least try and see what it would look like :)
I think it's perfectly fine to leave mwait always implemented as NOP - it's valid behavior.
As for the CPUID exposure, that should be a pure QEMU thing. If overriding CPUID bits the kernel mask tells us doesn't work today, we should just make it possible :).
Eventually I really think that -cpu foo,+mwait,+monitor or whatever the bits are should override any safety net that KVM gives us on features it thinks are safe to use.
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