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| author | Jiaming Zhang <r772577952@gmail.com> | 2025-09-06 01:47:36 +0800 |
|---|---|---|
| committer | Sean Christopherson <seanjc@google.com> | 2025-09-16 12:55:11 -0700 |
| commit | 5b5133e6a55bf6ca18de1b33c2ffe17d1162ec8e (patch) | |
| tree | 9ad02826a7af91b9d04d3e4ea8ca7bee23af80e2 | |
| parent | cbd860293d139fae44b76d821d8112aab89d8e17 (diff) | |
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 <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>
| -rw-r--r-- | Documentation/virt/kvm/api.rst | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 6aa40ee05a4a..b3f2395a62d1 100644 --- a/Documentation/virt/kvm/api.rst +++ b/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. |