linux-kernel - Re: [PATCH] KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status

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Message-ID: <Zz0EjiFMTiHWeq8h@linux.dev>
Date: Tue, 19 Nov 2024 13:35:10 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Raghavendra Rao Ananta <rananta@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Ignore PMCNTENSET_EL0 while checking for
 overflow status

Hi Raghu,

Thanks for finding this!

On Tue, Nov 19, 2024 at 08:58:41PM +0000, Raghavendra Rao Ananta wrote:
> kvm_pmu_overflow_status() currently checks if the PMCs are enabled for
> evaluating the PMU overflow condition. However, ARM ARM D13.1.1 states
> that a global enable control (PMCR.E), PMOVSSET<n>, and PMINTENSET<n>
> are sufficent to consider that the overflow condition is met. Hence,
> ignore the check for PMCNTENSET<n>.

It's more than sufficient, evaluating E, PMOVSSET<n>, and PMINTENSET<n>
is the *only* correct implementation of the architecture.

Also, ARM ARM section numbering is subject to change between revisions,
so it's always best to use a fully-qualified citation, like 'DDI0487K
D13.1.1'.

So I may rewrite this as:

  DDI0487K D13.1.1 describes the PMU overflow condition, which evaluates
  to true if any counter's global enable (PMCR_EL0.E), overflow flag
  (PMOVSSET_EL0[n]), and interrupt enable (PMINTENSET_EL1[n]) are all 1.
  Of note, this does not require a counter to be enabled (i.e.
  PMCNTENSET_EL0[n] = 1) to generate an overflow.

  Align kvm_pmu_overflow_status() with the reality of the architecture
  and stop using PMCNTENSET_EL0 as part of the overflow condition.

We've got yet another bug lurking here as of 6.13, since the hypervisor
range of counters isn't observing the correct global enable
(MDCR_EL2.HPME).

Let me fiddle with this and send out a combined set of fixes.

> The bug was discovered while running the SBSA PMU test, which only sets
> PMCR.E, PMOVSSET<0>, PMINTENSET<0>, and expects an overflow interrupt.
> 

We should be sending this to stable too.

Cc: stable@...r.kernel.org
Fixes: 76d883c4e640 ("arm64: KVM: Add access handler for PMOVSSET and PMOVSCLR register")

-- 
Thanks,
Oliver