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Message-ID: <0c5fb304-8c69-80c3-6f1e-487828554244@os.amperecomputing.com>
Date: Thu, 17 Aug 2023 14:57:55 +0530
From: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To: Marc Zyngier <maz@...nel.org>
Cc: linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
linux-arm-kernel@...ts.infradead.org, eauger@...hat.com,
miguel.luis@...cle.com, darren@...amperecomputing.com,
scott@...amperecomputing.com,
Christoffer Dall <Christoffer.Dall@....com>
Subject: Re: [PATCH 2/2] KVM: arm64: timers: Adjust CVAL of a ptimer across
guest entry and exits
Hi Marc,
On 17-08-2023 01:57 pm, Marc Zyngier wrote:
> [Fixing Christoffer's email address]
Thanks.
>
> On Thu, 17 Aug 2023 07:03:14 +0100,
> Ganapatrao Kulkarni <gankulkarni@...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.
As per ARM ARM (ARM DDI 0487J.a page D11-5989)
"When FEAT_ECV is implemented, the CNTPOFF_EL2 register allows an offset
to be applied to the physical counter, as viewed from EL1 and EL0, and
to the EL1 physical timer. The functionality of this 64-bit register is
affected by CNTHCTL_EL2.ECV."
As per ARM ARM (ARM DDI 0487J.a page D19-7857)
"When HCR_EL2.{E2H, TGE} == {1, 1} or SCR_EL3.{NS, EEL2} == {0, 0}, then
Enhanced Counter Virtualization functionality is disabled."
"The EL1 physical timer interrupt is triggered when ((PCount<63:0> -
CNTPOFF_EL2<63:0>) - PCVal<63:0>) is greater than or equal to 0."
As per ARM ARM (ARM DDI 0487J.a page D19-7938)
"When EL2 is implemented and enabled in the current Security state, the
physical counter uses a fixed physical offset of *zero* if any of the
following are true:
• CNTHCTL_EL2.ECV is 0.
• SCR_EL3.ECVEn is 0.
• HCR_EL2.{E2H, TGE} is {1, 1}."
In VHE host hypervisor, E2H=TGE=1 hence ECV is disabled and Ptimer
interrupt is triggered based on PCount<63:0> - PCVal<63:0>
Since cval is set by Guest as per offsetted PCounter value and pCount is
not subtracted by CNTPOFF when in VHE-L0, results in cval becoming much
lesser than physical counter(bumped up since CNTPOFF is zero) and timer
interrupt trigger condition is met falsely.
There is no issue/impact on cval due to ECV, however it can be/is
manipulated to handle this on and off of CNTPOFF/ECV.
IIUC, CNTPOFF and CNTVOFF are not same as per specification.
>
> 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.
>
Thanks,
Ganapat
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