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| author |   Pierre Morel <pmorel@linux.ibm.com> | 2020-09-07 15:26:07 +0200 |
|---|---|---|
| committer |   Christian Borntraeger <borntraeger@de.ibm.com> | 2021-03-09 10:16:26 +0100 |
| commit | 87e28a15c42cc592009c32a8c20e5789059027c2 (patch) | |
| tree | d1fecf787809e547737239ce637072fcca473c18 /Documentation/virt | |
| parent | KVM: s390: Fix comment spelling in kvm_s390_vcpu_start() (diff) | |
| download | linux-dev-87e28a15c42cc592009c32a8c20e5789059027c2.tar.xz linux-dev-87e28a15c42cc592009c32a8c20e5789059027c2.zip | |
KVM: s390: diag9c (directed yield) forwarding
When we intercept a DIAG_9C from the guest we verify that the
target real CPU associated with the virtual CPU designated by
the guest is running and if not we forward the DIAG_9C to the
target real CPU.
To avoid a diag9c storm we allow a maximal rate of diag9c forwarding.
The rate is calculated as a count per second defined as a new
parameter of the s390 kvm module: diag9c_forwarding_hz .
The default value of 0 is to not forward diag9c.
Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Link: https://lore.kernel.org/r/1613997661-22525-2-git-send-email-pmorel@linux.ibm.com
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Diffstat (limited to 'Documentation/virt')
| -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. |