linux-kernel - Re: [PATCH v6 4/9] KVM: arm64: Add vendor hypervisor firmware register

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Date:   Mon, 25 Apr 2022 10:12:32 -0700
From:   Oliver Upton <oupton@...gle.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Reiji Watanabe <reijiw@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v6 4/9] KVM: arm64: Add vendor hypervisor firmware register

On Mon, Apr 25, 2022 at 9:52 AM Raghavendra Rao Ananta
<rananta@...gle.com> wrote:

[...]

> > > +#define KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_MAX    1
> >
> > Nit: IMHO perhaps it might be more convenient to define the MAX macro
> > in arch/arm64/include/uapi/asm/kvm.h like below for maintenance ?
> > (The same comments are applied to other KVM_REG_ARM_*_BMAP_BIT_MAX)
> >
> > #define KVM_REG_ARM_VENDOR_HYP_BIT_MAX KVM_REG_ARM_VENDOR_HYP_BIT_PTP
> >
> We have been going back and forth on this :)
> It made sense for me to keep it in uapi as well, but I took Oliver's
> suggestion of keeping it outside of uapi since this is something that
> could be constantly changing [1].

The maximum set of features in a given bitmap register is a property
of the running system, not the headers chosen at compile time. There
is an illusion of ABI breakage when adding new bits to the registers
if we've declared the max bit in UAPI. We also define
KVM_VCPU_MAX_FEATURES outside of UAPI, even though it is related to
the KVM_ARM_VCPU_INIT ioctl.

--
Thanks,
Oliver