linux-kernel - Re: [PATCH v7 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR

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Date:   Thu, 19 Oct 2023 12:45:30 +0200 (CEST)
From:   Sebastian Ott <sebott@...hat.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
cc:     Oliver Upton <oliver.upton@...ux.dev>,
        Marc Zyngier <maz@...nel.org>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Shaoqin Huang <shahuang@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 08/12] KVM: arm64: PMU: Allow userspace to limit
 PMCR_EL0.N for the guest

On Tue, 17 Oct 2023, Raghavendra Rao Ananta wrote:
> On Tue, Oct 17, 2023 at 8:52 AM Sebastian Ott <sebott@...hat.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.

Yes I get that part. What I've meant is why specifically mention -EBUSY?
You're also not returning -EFAULT nor -EINVAL.

/*
  * Make PMCR immutable once the VM has started running, but do
  * not return an error to meet the existing expectations.
  */
IMHO provides the same info to the reader and is less confusing

Sebastian