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From:   Oliver Upton <oliver.upton@linux.dev>
To:     Marc Zyngier <maz@kernel.org>
Cc:     kvmarm@lists.linux.dev, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Quentin Perret <qperret@google.com>,
        Will Deacon <will@kernel.org>, Fuad Tabba <tabba@google.com>
Subject: Re: [PATCH v2 02/17] arm64: Prevent the use of
 is_kernel_in_hyp_mode() in hypervisor code
Message-ID: <ZHZUi/4kXxRmCa7a@linux.dev>
References: <20230526143348.4072074-1-maz@kernel.org>
 <20230526143348.4072074-3-maz@kernel.org>
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Hi Marc,

On Fri, May 26, 2023 at 03:33:33PM +0100, Marc Zyngier wrote:
> Using is_kernel_in_hyp_mode() in hypervisor code is a pretty bad
> mistake. This helper only checks for CurrentEL being EL2, which
> is always true.
> 
> Make the link fail if using the helper in hypervisor context
> by referencing a non-existent function. Whilst we're at it,
> flag the helper as __always_inline, which it really should be.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/virt.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 4eb601e7de50..91029709d133 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -110,8 +110,13 @@ static inline bool is_hyp_mode_mismatched(void)
>  	return __boot_cpu_mode[0] != __boot_cpu_mode[1];
>  }
>  
> -static inline bool is_kernel_in_hyp_mode(void)
> +extern void gotcha_is_kernel_in_hyp_mode(void);
> +
> +static __always_inline bool is_kernel_in_hyp_mode(void)
>  {
> +#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
> +	gotcha_is_kernel_in_hyp_mode();
> +#endif
>  	return read_sysreg(CurrentEL) == CurrentEL_EL2;
>  }

Would BUILD_BUG() work in this context, or have I missed something?

-- 
Thanks,
Oliver