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

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Message-ID: <8634x522z2.wl-maz@kernel.org>
Date:   Thu, 16 Nov 2023 14:21:37 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Jianyong Wu <jianyong.wu@....com>, james.morse@....com,
        will@...nel.org, rmk@...linux.org.uk, salil.mehta@...wei.com,
        suzuki.poulose@....com, oliver.upton@...ux.dev,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, 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.

	M.

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