Host latency peaks due to kvm-intel

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi,

did anyone recently tried current KVM for Intel over some real-time
Linux? I'm seeing more than 500 us latency peaks on the host,
specifically during VM startup. This applies to both 2.6.29.6-rt23 and
Xenomai/I-pipe. For -rt, I both tried the included (patched) KVM modules
as well as kvm.git head with some additionally required -rt fixes.
Xenomai ran over a 2.6.30 kernel with my own KVM-enabler patch.

Early instrumentation actually points to the guest exit itself: I added
markers right before and after the assembly part of vmx_vcpu_run, and
further instrumentation reports that the next host APIC tick should go
off right inside guest mode. But KVM leaves the switching part 500 us
too late in that case - as if guest exit on external IRQs was disabled.

Will debug this further, but I'm also curious to hear other user
experiences.

Jan

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Siemens AG, Corporate Technology, CT SE 2
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