Re: [PATCH v7 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

From: Raghavendra Rao Ananta <rananta@google.com>
To: Sebastian Ott <sebott@redhat.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	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 v7 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest
Date: Tue, 17 Oct 2023 09:49:07 -0700	[thread overview]
Message-ID: <CAJHc60z7U1-irTy-6URb_V0PTW+TYS4qodf2akSg33_7CJgjyw@mail.gmail.com> (raw)
In-Reply-To: <5d35c9f3-455e-6aa9-fd6a-4433cf70803a@redhat.com>

On Tue, Oct 17, 2023 at 8:52 AM Sebastian Ott <sebott@redhat.com> wrote:
>
> On Mon, 9 Oct 2023, Raghavendra Rao Ananta wrote:
> > +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> > +                 u64 val)
> > +{
> > +     struct kvm *kvm = vcpu->kvm;
> > +     u64 new_n, mutable_mask;
> > +
> > +     mutex_lock(&kvm->arch.config_lock);
> > +
> > +     /*
> > +      * Make PMCR immutable once the VM has started running, but do
> > +      * not return an error (-EBUSY) to meet the existing expectations.
> > +      */
>
> Why should we mention which error we're _not_ returning?
>
Oh, it's not to break the existing userspace expectations. Before this
series, any 'write' from userspace was possible. Returning -EBUSY all
of a sudden might tamper with this expectation.

Thank you.
Raghavendra
>
> > +     if (kvm_vm_has_ran_once(vcpu->kvm)) {
> > +             mutex_unlock(&kvm->arch.config_lock);
> > +             return 0;
> > +     }
>