kvm: reuse per-vcpu stats fd to avoid vcpu interruption

Marcelo Tosattirequested to merge
mtosatti/qemu-kvm:qemusigipi into c9s

Merge Request Required Information

kvm: reuse per-vcpu stats fd to avoid vcpu interruption

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2218644
Commit: 3b6f485275ae95a81eec589d2773b86ca9ddec4d

A regression has been detected in latency testing of KVM guests.
More specifically, it was observed that the cyclictest
numbers inside of an isolated vcpu (running on isolated pcpu) are:

Where a maximum of 50us is acceptable.

The implementation of KVM_GET_STATS_FD uses run_on_cpu to query
per vcpu statistics, which interrupts the vcpu (and is unnecessary).

To fix this, open the per vcpu stats fd on vcpu initialization,
and read from that fd from QEMU's main thread.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Summary of Changes

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