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

[Jianyong Wu]

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