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From:   Cornelia Huck <cohuck@redhat.com>
To:     Jianyong Wu <jianyong.wu@arm.com>, maz@kernel.org,
        james.morse@arm.com, will@kernel.org
Cc:     rmk@armlinux.org.uk, salil.mehta@huawei.com,
        suzuki.poulose@arm.com, oliver.upton@linux.dev,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-kernel@vger.kernel.org, justin.he@arm.com,
        jianyong.wu@arm.com
Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
In-Reply-To: <20231116114152.912344-1-jianyong.wu@arm.com>
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References: <20231116114152.912344-1-jianyong.wu@arm.com>
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Date:   Thu, 16 Nov 2023 14:08:58 +0100
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On Thu, Nov 16 2023, Jianyong Wu <jianyong.wu@arm.com> wrote:

> 821d935c87b introduces support for userspace SMCCC filtering, but lack
> of a way to tell userspace if we have this feature. Add a corresponding
> feature extension can resolve this issue.
>
> For example, the incoming feature Vcpu Hotplug needs the SMCCC filter.
> As there is no way to check this feature, VMM will run into error when
> it calls this feature on an old kernel. It's bad for backward compatible.

Can't you simply query via KVM_HAS_DEVICE_ATTR whether the SMCCC
filtering controls exist?

>
> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> ---
>  Documentation/virt/kvm/api.rst | 3 ++-
>  arch/arm64/kvm/arm.c           | 1 +
>  include/uapi/linux/kvm.h       | 1 +
>  3 files changed, 4 insertions(+), 1 deletion(-)
>