Documentation: KVM: Call out that KVM strictly follows the 8254 PIT spec

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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[sean: add context Link, drop local APIC change, massage changelog accordingly]
Signed-off-by: Sean Christopherson <[email protected]>

Author: Radon10043

Documentation/virt/kvm/api.rst

@@ -3075,6 +3075,12 @@ This IOCTL replaces the obsolete KVM_GET_PIT.
 Sets the state of the in-kernel PIT model. Only valid after KVM_CREATE_PIT2.
 See KVM_GET_PIT2 for details on struct kvm_pit_state2.
 
+.. Tip::
+  ``KVM_SET_PIT2`` strictly adheres to the spec of Intel 8254 PIT.  For example,
+  a ``count`` value of 0 in ``struct kvm_pit_channel_state`` is interpreted as
+  65536, which is the maximum count value. Refer to `Intel 8254 programmable
+  interval timer <https://www.scs.stanford.edu/10wi-cs140/pintos/specs/8254.pdf>`_.
+
 This IOCTL replaces the obsolete KVM_SET_PIT.