On 2023-02-08 at 01:04:19 +0000, "Michael Kelley (LINUX)" <mikelley@xxxxxxxxxxxxx> said: > From: lirongqing@xxxxxxxxx <lirongqing@xxxxxxxxx> Sent: Monday, February 6, 2023 5:15 PM > > > > Zeroing the counter register in pit_shutdown() isn't actually supposed to > > stop it from counting, will causes the PIT to start running again, > > From the spec: > > > > The largest possible initial count is 0; this is equivalent to 216 for > > binary counting and 104 for BCD counting. > > > > The Counter does not stop when it reaches zero. In Modes 0, 1, 4, and 5 the > > Counter "wraps around" to the highest count, either FFFF hex for binary > > count- ing or 9999 for BCD counting, and continues counting. > > > > Mode 0 is typically used for event counting. After the Control Word is > > written, OUT is initially low, and will remain low until the Counter > > reaches zero. OUT then goes high and remains high until a new count or a > > new Mode 0 Control Word is written into the Counter. > > > > Hyper-V and KVM follow the spec, the issue that 35b69a42 "(clockevents/drivers/ > > i8253: Add support for PIT shutdown quirk") fixed is in i8253 drivers, not Hyper-v, > > so delete the zero timer counter register in shutdown, and delete PIT shutdown > > quirk for Hyper-v > > From the standpoint of Hyper-V, I'm good with this change. But there's a > risk that old hardware might not be compliant with the spec, and needs the > zero'ing for some reason. The experts in the x86 space will be in the best > position to assess the risk. At the time, the quirk approach was taken so > the change applied only to Hyper-V, and any such risk was avoided. > > For Hyper-V, > Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx> It's not entirely clear why we zero it at all. What was it supposed to achieve? I suppose if we want to be ultra-careful, perhaps we could do the zeroing only on real hardware, if hypervisor_is_type(X86_HYPER_NATIVE)? This does seem to be making VMMs spend a bunch of time stealing cycles from the guest to emulate an unwanted timer, so it would be good to fix it.
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