+++ /dev/null
-.. SPDX-License-Identifier: GPL-2.0
-
-===========================
-The KVM halt polling system
-===========================
-
-The KVM halt polling system provides a feature within KVM whereby the latency
-of a guest can, under some circumstances, be reduced by polling in the host
-for some time period after the guest has elected to no longer run by cedeing.
-That is, when a guest vcpu has ceded, or in the case of powerpc when all of the
-vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
-before giving up the cpu to the scheduler in order to let something else run.
-
-Polling provides a latency advantage in cases where the guest can be run again
-very quickly by at least saving us a trip through the scheduler, normally on
-the order of a few micro-seconds, although performance benefits are workload
-dependant. In the event that no wakeup source arrives during the polling
-interval or some other task on the runqueue is runnable the scheduler is
-invoked. Thus halt polling is especially useful on workloads with very short
-wakeup periods where the time spent halt polling is minimised and the time
-savings of not invoking the scheduler are distinguishable.
-
-The generic halt polling code is implemented in:
-
- virt/kvm/kvm_main.c: kvm_vcpu_block()
-
-The powerpc kvm-hv specific case is implemented in:
-
- arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked()
-
-Halt Polling Interval
-=====================
-
-The maximum time for which to poll before invoking the scheduler, referred to
-as the halt polling interval, is increased and decreased based on the perceived
-effectiveness of the polling in an attempt to limit pointless polling.
-This value is stored in either the vcpu struct:
-
- kvm_vcpu->halt_poll_ns
-
-or in the case of powerpc kvm-hv, in the vcore struct:
-
- kvmppc_vcore->halt_poll_ns
-
-Thus this is a per vcpu (or vcore) value.
-
-During polling if a wakeup source is received within the halt polling interval,
-the interval is left unchanged. In the event that a wakeup source isn't
-received during the polling interval (and thus schedule is invoked) there are
-two options, either the polling interval and total block time[0] were less than
-the global max polling interval (see module params below), or the total block
-time was greater than the global max polling interval.
-
-In the event that both the polling interval and total block time were less than
-the global max polling interval then the polling interval can be increased in
-the hope that next time during the longer polling interval the wake up source
-will be received while the host is polling and the latency benefits will be
-received. The polling interval is grown in the function grow_halt_poll_ns() and
-is multiplied by the module parameters halt_poll_ns_grow and
-halt_poll_ns_grow_start.
-
-In the event that the total block time was greater than the global max polling
-interval then the host will never poll for long enough (limited by the global
-max) to wakeup during the polling interval so it may as well be shrunk in order
-to avoid pointless polling. The polling interval is shrunk in the function
-shrink_halt_poll_ns() and is divided by the module parameter
-halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0.
-
-It is worth noting that this adjustment process attempts to hone in on some
-steady state polling interval but will only really do a good job for wakeups
-which come at an approximately constant rate, otherwise there will be constant
-adjustment of the polling interval.
-
-[0] total block time:
- the time between when the halt polling function is
- invoked and a wakeup source received (irrespective of
- whether the scheduler is invoked within that function).
-
-Module Parameters
-=================
-
-The kvm module has 3 tuneable module parameters to adjust the global max
-polling interval as well as the rate at which the polling interval is grown and
-shrunk. These variables are defined in include/linux/kvm_host.h and as module
-parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
-powerpc kvm-hv case.
-
-+-----------------------+---------------------------+-------------------------+
-|Module Parameter | Description | Default Value |
-+-----------------------+---------------------------+-------------------------+
-|halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT|
-| | interval which defines | |
-| | the ceiling value of the | |
-| | polling interval for | (per arch value) |
-| | each vcpu. | |
-+-----------------------+---------------------------+-------------------------+
-|halt_poll_ns_grow | The value by which the | 2 |
-| | halt polling interval is | |
-| | multiplied in the | |
-| | grow_halt_poll_ns() | |
-| | function. | |
-+-----------------------+---------------------------+-------------------------+
-|halt_poll_ns_grow_start| The initial value to grow | 10000 |
-| | to from zero in the | |
-| | grow_halt_poll_ns() | |
-| | function. | |
-+-----------------------+---------------------------+-------------------------+
-|halt_poll_ns_shrink | The value by which the | 0 |
-| | halt polling interval is | |
-| | divided in the | |
-| | shrink_halt_poll_ns() | |
-| | function. | |
-+-----------------------+---------------------------+-------------------------+
-
-These module parameters can be set from the debugfs files in:
-
- /sys/module/kvm/parameters/
-
-Note: that these module parameters are system wide values and are not able to
- be tuned on a per vm basis.
-
-Further Notes
-=============
-
-- Care should be taken when setting the halt_poll_ns module parameter as a large value
- has the potential to drive the cpu usage to 100% on a machine which would be almost
- entirely idle otherwise. This is because even if a guest has wakeups during which very
- little work is done and which are quite far apart, if the period is shorter than the
- global max polling interval (halt_poll_ns) then the host will always poll for the
- entire block time and thus cpu utilisation will go to 100%.
-
-- Halt polling essentially presents a trade off between power usage and latency and
- the module parameters should be used to tune the affinity for this. Idle cpu time is
- essentially converted to host kernel time with the aim of decreasing latency when
- entering the guest.
-
-- Halt polling will only be conducted by the host when no other tasks are runnable on
- that cpu, otherwise the polling will cease immediately and schedule will be invoked to
- allow that other task to run. Thus this doesn't allow a guest to denial of service the
- cpu.
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