linux-kernel - Re: [PATCH v5 02/12] KVM: arm64: PMU: Set the default PMU for the guest on vCPU reset

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Date:   Mon, 18 Sep 2023 09:58:31 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     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 v5 02/12] KVM: arm64: PMU: Set the default PMU for the
 guest on vCPU reset

On Mon, Sep 18, 2023 at 9:47 AM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> On Mon, Sep 18, 2023 at 09:41:02AM -0700, Raghavendra Rao Ananta wrote:
> > On Fri, Sep 15, 2023 at 12:33 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> [...]
>
> > > This would eliminate the possibility of returning ENODEV to userspace
> > > where we shouldn't.
> > >
> > I understand that we'll be breaking the API contract and userspace may
> > have to adapt to this change, but is it not acceptable to document and
> > return ENODEV, since ENODEV may offer more clarity to userspace as to
> > why the ioctl failed? In general, do we never extend the APIs?
>
> Yes, we extend the existing interfaces all the time, but we almost
> always require user opt in for user-visible changes in behavior. Look at
> the way arm64_check_features() is handled -- we hide the 'detailed'
> error and return EINVAL due to UAPI.
>
Got it. Let's return EINVAL then. Thanks!

- Raghavendra
> --
> Thanks,
> Oliver