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From:   Salil Mehta <salil.mehta@huawei.com>
To:     Cornelia Huck <cohuck@redhat.com>,
        Jianyong Wu <jianyong.wu@arm.com>,
        "maz@kernel.org" <maz@kernel.org>,
        "james.morse@arm.com" <james.morse@arm.com>,
        "will@kernel.org" <will@kernel.org>
CC:     "rmk@armlinux.org.uk" <rmk@armlinux.org.uk>,
        "suzuki.poulose@arm.com" <suzuki.poulose@arm.com>,
        "oliver.upton@linux.dev" <oliver.upton@linux.dev>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "kvmarm@lists.linux.dev" <kvmarm@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "justin.he@arm.com" <justin.he@arm.com>,
        "jianyong.wu@arm.com" <jianyong.wu@arm.com>
Subject: RE: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
Thread-Topic: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
Thread-Index: AQHaGIJSPxqmEiVqkkOrXIqhoEeoZ7B86xkAgAAOImA=
Date:   Thu, 16 Nov 2023 14:06:03 +0000
Message-ID: <2ea495b04a7d484aa570059298e20862@huawei.com>
References: <20231116114152.912344-1-jianyong.wu@arm.com>
 <877cmhq1zp.fsf@redhat.com>
In-Reply-To: <877cmhq1zp.fsf@redhat.com>
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> From: Cornelia Huck <cohuck@redhat.com>
> Sent: Thursday, November 16, 2023 1:09 PM
> To: Jianyong Wu <jianyong.wu@arm.com>; maz@kernel.org; james.morse@arm.com;
> will@kernel.org
> 
> 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?


Agreed. In fact, this is what I had earlier intended to do but deferred this
change. As of now, RFC V2 of vCPU Hotplug series does not have this check yet
while installing the SMCCC filters in KVM Host.

Thanks

> > 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(-)
> >