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Explicitly document that the behavior of KVM_SET_PIT2 strictly conforms to the Intel 8254 PIT hardware specification, specifically that a write of '0' adheres to the spec's definition that a programmed count of '0' is converted to the maximum possible value (2^16). E.g. an unaware userspace might attempt to validate that KVM_GET_PIT2 returns the exact state set via KVM_SET_PIT2, and be surprised when the returned count is 65536, not 0. Add a references to the Intel 8254 PIT datasheet that will hopefully stay fresh for some time (the internet isn't exactly brimming with copies of the 8254 datasheet). Link: https://lore.kernel.org/all/CANypQFbEySjKOFLqtFFf2vrEe=NBr7XJfbkjQhqXuZGg7Rpoxw@mail.gmail.com Signed-off-by: Jiaming Zhang <r772577952@gmail.com> Link: https://lore.kernel.org/r/20250905174736.260694-1-r772577952@gmail.com [sean: add context Link, drop local APIC change, massage changelog accordingly] Signed-off-by: Sean Christopherson <seanjc@google.com>