From: Marc Zyngier <maz@kernel.org>
To: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Cc: linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev,
linux-arm-kernel@lists.infradead.org, eauger@redhat.com,
miguel.luis@oracle.com, darren@os.amperecomputing.com,
scott@os.amperecomputing.com,
Christoffer Dall <Christoffer.Dall@arm.com>
Subject: Re: [PATCH 2/2] KVM: arm64: timers: Adjust CVAL of a ptimer across guest entry and exits
Date: Thu, 17 Aug 2023 09:27:38 +0100 [thread overview]
Message-ID: <87bkf6oyyt.wl-maz@kernel.org> (raw)
In-Reply-To: <20230817060314.535987-3-gankulkarni@os.amperecomputing.com>
[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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next prev parent reply other threads:[~2023-08-17 8:28 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-17 6:03 [PATCH 0/2] Avoid spurious ptimer interrupts for non-zero cntpoff Ganapatrao Kulkarni
2023-08-17 6:03 ` [PATCH 1/2] KVM: arm64: timers: Move helper has_cntpoff to a header file Ganapatrao Kulkarni
2023-08-17 6:03 ` [PATCH 2/2] KVM: arm64: timers: Adjust CVAL of a ptimer across guest entry and exits Ganapatrao Kulkarni
2023-08-17 8:27 ` Marc Zyngier [this message]
2023-08-17 9:27 ` Ganapatrao Kulkarni
2023-08-17 10:04 ` Marc Zyngier
2023-08-22 12:16 ` Marc Zyngier
2023-08-22 14:57 ` Ganapatrao Kulkarni
2023-08-24 6:37 ` Ganapatrao Kulkarni
2023-08-28 10:17 ` Marc Zyngier
2023-09-01 12:15 ` Ganapatrao Kulkarni
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