linux-kernel - Re: [PATCH v5 21/24] KVM: arm64: Inject recorded guest interrupts

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Message-ID: <aUBKX8iqsptyTb4S@kernel.org>
Date: Mon, 15 Dec 2025 09:50:23 -0800
From: Oliver Upton <oupton@...nel.org>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, corbet@....net,
	linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
	maz@...nel.org, oliver.upton@...ux.dev, mizhang@...gle.com,
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
	mark.rutland@....com, shuah@...nel.org,
	gankulkarni@...amperecomputing.com, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, linux-perf-users@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 21/24] KVM: arm64: Inject recorded guest interrupts

On Fri, Dec 12, 2025 at 10:55:06PM +0000, Colton Lewis wrote:
> Oliver Upton <oupton@...nel.org> writes:
> 
> > In no situation should KVM be injecting a "recorded" IRQ. The overflow
> > condition of the PMU is well defined in the architecture and we should
> > implement *exactly* that.
> 
> When I say "record" I just meant "updating the virtual overflow register
> to reflect an overflow".

Right, consider changing the shortlog to read more along the lines of
"detect overflows for partitioned PMU" or similar.

> > On Tue, Dec 09, 2025 at 08:51:18PM +0000, Colton Lewis wrote:
> > > +/**
> > > + * kvm_pmu_part_overflow_status() - Determine if any guest counters
> > > have overflowed
> > > + * @vcpu: Ponter to struct kvm_vcpu
> > > + *
> > > + * Determine if any guest counters have overflowed and therefore an
> > > + * IRQ needs to be injected into the guest.
> > > + *
> > > + * Return: True if there was an overflow, false otherwise
> > > + */
> > > +bool kvm_pmu_part_overflow_status(struct kvm_vcpu *vcpu)
> > > +{
> > > +	struct arm_pmu *pmu = vcpu->kvm->arch.arm_pmu;
> > > +	u64 mask = kvm_pmu_guest_counter_mask(pmu);
> > > +	u64 pmovs = __vcpu_sys_reg(vcpu, PMOVSSET_EL0);
> > > +	u64 pmint = read_pmintenset();
> > > +	u64 pmcr = read_pmcr();
> 
> > How do we know that the vPMU has been loaded on the CPU at this point?
> 
> Because this is only called by kvm_pmu_update_state which is only called
> by kvm_pmu_update_state <- kvm_pmu_{flush,sync}_hwstate <-
> kvm_arch_vcpu_ioctl_run after a vcpu_load.

That's assuming the PMU is loaded eagerly which I thought we agreed it
would not be.

Thanks,
Oliver