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Diffstat (limited to 'Documentation/virt/kvm/s390-diag.rst')
| -rw-r--r-- | Documentation/virt/kvm/s390-diag.rst | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/s390-diag.rst b/Documentation/virt/kvm/s390-diag.rst index eaac4864d3d6..ca85f030eb0b 100644 --- a/Documentation/virt/kvm/s390-diag.rst +++ b/Documentation/virt/kvm/s390-diag.rst @@ -84,3 +84,36 @@ If the function code specifies 0x501, breakpoint functions may be performed. This function code is handled by userspace. This diagnose function code has no subfunctions and uses no parameters. + + +DIAGNOSE function code 'X'9C - Voluntary Time Slice Yield +--------------------------------------------------------- + +General register 1 contains the target CPU address. + +In a guest of a hypervisor like LPAR, KVM or z/VM using shared host CPUs, +DIAGNOSE with function code 0x9c may improve system performance by +yielding the host CPU on which the guest CPU is running to be assigned +to another guest CPU, preferably the logical CPU containing the specified +target CPU. + + +DIAG 'X'9C forwarding ++++++++++++++++++++++ + +The guest may send a DIAGNOSE 0x9c in order to yield to a certain +other vcpu. An example is a Linux guest that tries to yield to the vcpu +that is currently holding a spinlock, but not running. + +However, on the host the real cpu backing the vcpu may itself not be +running. +Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to +the backing cpu will hopefully cause that cpu, and thus subsequently +the guest's vcpu, to be scheduled. + + +diag9c_forwarding_hz + KVM kernel parameter allowing to specify the maximum number of DIAGNOSE + 0x9c forwarding per second in the purpose of avoiding a DIAGNOSE 0x9c + forwarding storm. + A value of 0 turns the forwarding off. |