linux-kernel - RE: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter

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Date:   Tue, 21 Nov 2023 02:01:02 +0000
From:   Jianyong Wu <Jianyong.Wu@....com>
To:     Marc Zyngier <maz@...nel.org>, Cornelia Huck <cohuck@...hat.com>
CC:     James Morse <James.Morse@....com>,
        "will@...nel.org" <will@...nel.org>,
        "rmk@...linux.org.uk" <rmk@...linux.org.uk>,
        "salil.mehta@...wei.com" <salil.mehta@...wei.com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        "oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Justin He <Justin.He@....com>
Subject: RE: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter



> -----Original Message-----
> From: Marc Zyngier <maz@...nel.org>
> Sent: 2023年11月16日 22:22
> To: Cornelia Huck <cohuck@...hat.com>
> Cc: Jianyong Wu <Jianyong.Wu@....com>; James Morse
> <James.Morse@....com>; will@...nel.org; rmk@...linux.org.uk;
> salil.mehta@...wei.com; Suzuki Poulose <Suzuki.Poulose@....com>;
> oliver.upton@...ux.dev; linux-arm-kernel@...ts.infradead.org;
> kvmarm@...ts.linux.dev; linux-kernel@...r.kernel.org; Justin He
> <Justin.He@....com>
> Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
> 
> On Thu, 16 Nov 2023 13:08:58 +0000,
> Cornelia Huck <cohuck@...hat.com> wrote:
> >
> > On Thu, Nov 16 2023, Jianyong Wu <jianyong.wu@....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.

Exactly. Commit e0fc6b21616dd has done for this. Ignore this patch.

Thanks
Jianyong
> 
> 	M.
> 
> --
> Without deviation from the norm, progress is not possible.