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From: Oliver Upton <oliver.upton@linux.dev>
To: Raghavendra Rao Ananta <rananta@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Shaoqin Huang <shahuang@redhat.com>,
	Jing Zhang <jingzhangos@google.com>,
	Reiji Watanabe <reijiw@google.com>,
	Colton Lewis <coltonlewis@google.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v8 07/13] KVM: arm64: PMU: Allow userspace to limit
 PMCR_EL0.N for the guest
Message-ID: <ZTgO3SHzvd_mQTI9@linux.dev>
References: <20231020214053.2144305-1-rananta@google.com>
 <20231020214053.2144305-8-rananta@google.com>
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On Fri, Oct 20, 2023 at 09:40:47PM +0000, Raghavendra Rao Ananta wrote:

[...]

> +	/*
> +	 * Make PMCR immutable once the VM has started running, but
> +	 * do not return an error to meet the existing expectations.
> +	 */
> +	if (kvm_vm_has_ran_once(vcpu->kvm)) {
> +		mutex_unlock(&kvm->arch.config_lock);
> +		return 0;
> +	}

Marc pointed out offline that PMCR_EL0 needs to be mutable at runtime as
well. I'll admit, the architecture isn't very helpful as it is both used
for identification _and_ configuration.

What I had in mind a few revisions ago when I gave the suggestion was to
ignore writes to _just_ the PMCR_EL0.N field after the VM has started.

-- 
Thanks,
Oliver