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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 v5 02/12] KVM: arm64: PMU: Set the default PMU for the
 guest on vCPU reset
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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@linux.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.

--
Thanks,
Oliver