Commit 34e30ebb authored by David Matlack's avatar David Matlack Committed by Paolo Bonzini
Browse files

KVM: Document the interaction between KVM_CAP_HALT_POLL and halt_poll_ns 



Clarify the existing documentation about how KVM_CAP_HALT_POLL and
halt_poll_ns interact to make it clear that VMs using KVM_CAP_HALT_POLL
ignore halt_poll_ns.

Signed-off-by: default avatarDavid Matlack <dmatlack@google.com>
Message-Id: <20221201195249.3369720-3-dmatlack@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent b8b43a4c

Documentation/virt/kvm/api.rst

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@@ -7213,14 +7213,13 @@ veto the transition. 
:Parameters: args[0] is the maximum poll time in nanoseconds

:Returns: 0 on success; -1 on error



This capability overrides the kvm module parameter halt_poll_ns for the

target VM.



VCPU polling allows a VCPU to poll for wakeup events instead of immediately

scheduling during guest halts. The maximum time a VCPU can spend polling is

controlled by the kvm module parameter halt_poll_ns. This capability allows

the maximum halt time to specified on a per-VM basis, effectively overriding

the module parameter for the target VM.

KVM_CAP_HALT_POLL overrides the kvm.halt_poll_ns module parameter to set the

maximum halt-polling time for all vCPUs in the target VM. This capability can

be invoked at any time and any number of times to dynamically change the

maximum halt-polling time.



See Documentation/virt/kvm/halt-polling.rst for more information on halt

polling.



7.21 KVM_CAP_X86_USER_SPACE_MSR

-------------------------------

Documentation/virt/kvm/halt-polling.rst

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@@ -119,6 +119,19 @@ These module parameters can be set from the debugfs files in: 
Note: that these module parameters are system wide values and are not able to

      be tuned on a per vm basis.



Any changes to these parameters will be picked up by new and existing vCPUs the

next time they halt, with the notable exception of VMs using KVM_CAP_HALT_POLL

(see next section).



KVM_CAP_HALT_POLL

=================



KVM_CAP_HALT_POLL is a VM capability that allows userspace to override halt_poll_ns

on a per-VM basis. VMs using KVM_CAP_HALT_POLL ignore halt_poll_ns completely (but

still obey halt_poll_ns_grow, halt_poll_ns_grow_start, and halt_poll_ns_shrink).



See Documentation/virt/kvm/api.rst for more information on this capability.



Further Notes

=============