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: David Matlack <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: dmatlack

Documentation/virt/kvm/api.rst

@@ -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

@@ -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
 =============