linux-kernel - Re: [PATCH v2 6/7] KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system

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Date:   Fri, 9 Sep 2022 11:15:47 +0100
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Reiji Watanabe <reijiw@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/7] KVM: arm64: Treat 32bit ID registers as RAZ/WI on
 64bit-only system

Hi Reiji,

On Tue, Sep 06, 2022 at 09:52:53PM -0700, Reiji Watanabe wrote:

[...]

> >         /* CRm=3 */
> > -       ID_SANITISED(MVFR0_EL1),
> > -       ID_SANITISED(MVFR1_EL1),
> > -       ID_SANITISED(MVFR2_EL1),
> > +       AA32_ID_SANITISED(MVFR0_EL1),
> > +       AA32_ID_SANITISED(MVFR1_EL1),
> > +       AA32_ID_SANITISED(MVFR2_EL1),
> >         ID_UNALLOCATED(3,3),
> > -       ID_SANITISED(ID_PFR2_EL1),
> > +       AA32_ID_SANITISED(ID_PFR2_EL1),
> >         ID_HIDDEN(ID_DFR1_EL1),
> 
> Perhaps it might be better to handle ID_AFR0_EL1 and ID_DFR1_EL1
> in the same way as the other AArch32 ID registers for consistency ?
> (i.e. treat them RAZ/USER_WI instead of RAZ if kvm_supports_32bit_el0()
>  is false instead of RAZ)

Thanks for having a look. I stopped short of treating these registers as
RAZ/USER_WI since an attempted nonzero write to either of these
registers is a userspace bug (KVM always advertised 0).

As the ABI isn't busted for these registers I'd prefer to leave it in
place. Having said that, I'm not too strongly motivated in either
direction.

--
Thanks,
Oliver