Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
Date:   Thu, 16 Nov 2023 14:21:37 +0000
Message-ID: <8634x522z2.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     Jianyong Wu <jianyong.wu@arm.com>, james.morse@arm.com,
        will@kernel.org, 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
Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
In-Reply-To: <877cmhq1zp.fsf@redhat.com>
References: <20231116114152.912344-1-jianyong.wu@arm.com>
        <877cmhq1zp.fsf@redhat.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/29.1
 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Thu, 16 Nov 2023 13:08:58 +0000,
Cornelia Huck <cohuck@redhat.com> wrote:
> 
> 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?

Quite. Commit e0fc6b21616dd introduced it for that exact purpose,
specifically to prevent adding more of these capabilities when there
is a corresponding attribute that can be readily queried.

	M.

-- 
Without deviation from the norm, progress is not possible.