Re: [Question] Regarding PMU initialization within the QEMU for ARM VCPUs

From: Andrew Jones <drjones@redhat.com>
To: Auger Eric <eric.auger@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>,
	Salil Mehta <salil.mehta@huawei.com>,
	"mst@redhat.com" <mst@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"qemu-arm@nongnu.org" <qemu-arm@nongnu.org>,
	Igor Mammedov <imammedo@redhat.com>
Subject: Re: [Question] Regarding PMU initialization within the QEMU for ARM VCPUs
Date: Wed, 3 Jun 2020 12:21:16 +0200	[thread overview]
Message-ID: <20200603102116.hbo4kmaitvh2kofl@kamzik.brq.redhat.com> (raw)
In-Reply-To: <fb9cf73c-e94b-b7f9-1d11-dc0353dd3996@redhat.com>

On Wed, Jun 03, 2020 at 11:59:23AM +0200, Auger Eric wrote:
> Hi Drew,
> 
> On 6/3/20 11:37 AM, Andrew Jones wrote:
> > On Mon, Jun 01, 2020 at 03:04:33PM +0000, Salil Mehta wrote:
> >> Hello,
> >> I could see below within function fdt_add_pmu_nodes() part of
> >> hw/arm/virt.c during virt machine initialization time:
> >>
> >> Observation:
> >> In below function, support of PMU feature is being checked for
> >> each vcpu and if the PMU is found part of the features then PMU
> >> is initialized with in the host/KVM. But if there is even one
> >> vcpu which is found to not support the PMU then loop is exited
> >> and PMU is not initialized for the rest of the vcpus as well.
> >>
> >> Questions:
> >> Q1. Not sure what is the logic of the premature exit and not
> >>     continuing with further checks and initialization of other
> >>     VCPU PMUs?
> > 
> > KVM requires all VCPUs to have a PMU if one does.
> 
> I fail to find where this is enforced? Do you know the place?

kvm_vcpu_set_target(), called from KVM_ARM_VCPU_INIT. It ensures
all VCPUs have identical features selected. The PMU requires a
VCPU feature (KVM_ARM_VCPU_PMU_V3) to be set.

Thanks,
drew