linux-kernel - Re: [PATCH] selftests: KVM: aarch64: Let hypercalls use UAPI *_BIT

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Date:   Wed, 4 May 2022 19:58:42 +0000
From:   Oliver Upton <oupton@...gle.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     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>,
        Reiji Watanabe <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests: KVM: aarch64: Let hypercalls use UAPI
 *_BIT_COUNT

Hi Raghavendra,

On Wed, May 04, 2022 at 06:44:15PM +0000, Raghavendra Rao Ananta wrote:
> The hypercalls test currently defines its own *_BMAP_BIT_MAX macros to
> define the last valid feature bit for each bitmap firmware register.
> However, since these definitions are already present in the uapi header,
> kvm.h, as *_BMAP_BIT_COUNT, and would help to keep the test updated as
> features grow, use these instead.

LOL, looks like I lost that one in the end! Still, the fact that you're
patching the selftest highlights the fact that there is a nonzero chance
of userspace using this value incorrectly expecting it to hold true
across all kernels.

Since this is the route going forward can we please consider documenting
the fact that _BIT_COUNT *will* change and is not stable between kernel
versions. Bad UAPI expectations could throw a wrench into this entire
plan we've hatched for preserving hypercall ABI.

Just a warning at the end of the register documentation would suffice.

> No functional change intended.
> 
> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>

Besides considering the above hazard:

Reviewed-by: Oliver Upton <oupton@...gle.com>