Messages in this thread | | | Date | Thu, 17 Aug 2023 09:27:38 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH 2/2] KVM: arm64: timers: Adjust CVAL of a ptimer across guest entry and exits |
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[Fixing Christoffer's email address]
On Thu, 17 Aug 2023 07:03:14 +0100, Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote: > > As per FEAT_ECV, when HCR_EL2.{E2H, TGE} == {1, 1}, Enhanced Counter > Virtualization functionality is disabled and CNTPOFF_EL2 value is treated > as zero. On VHE host, E2H and TGE are set, hence it is required > to adjust CVAL by incrementing it by CNTPOFF_EL2 after guest > exit to avoid false physical timer interrupts and also > decrement/restore CVAL before the guest entry.
No, this is wrong. Neither E2H nor TGE have any impact on writing to CNTPOFF_EL2, nor does it have an impact on CNTP_CVAL_EL0. Just read the pseudocode to convince yourself.
CNTPOFF_EL2 is applied at exactly two points: when SW is reading CNTPCT_EL0 from EL1 while {E2H,TGE}=={1, 0} and when the HW is comparing CNTPCT_EL0 with the CNTP_CVAL_EL0. In both cases the offset is subtracted from the counter. And that's the point where the running EL matters. Which means that CNTPOFF_EL2 behaves exactly like CNTVOFF_EL2. No ifs, no buts.
The behaviour you are describing tends to indicate that your HW is applying CNTPOFF_EL2 to CVAL, which would be a gold plated bug.
It doesn't mean that the KVM code is correct either. I'm seeing pretty bad behaviours on a machine without CNTPOFF_EL2. But what you are suggesting is IMO a misunderstanding of the architecture.
Thanks,
M.
-- Without deviation from the norm, progress is not possible.
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