Re: [PATCH 2/4] KVM: arm64: Extend comment in has_vhe()

From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu, Marc Zyngier <maz@kernel.org>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Fuad Tabba <tabba@google.com>,
	Quentin Perret <qperret@google.com>,
	Sean Christopherson <seanjc@google.com>,
	David Brazdil <dbrazdil@google.com>,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/4] KVM: arm64: Extend comment in has_vhe()
Date: Fri, 4 Jun 2021 15:09:09 +0100	[thread overview]
Message-ID: <20210604140909.GB69333@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210603183347.1695-3-will@kernel.org>

On Thu, Jun 03, 2021 at 07:33:45PM +0100, Will Deacon wrote:
> has_vhe() expands to a compile-time constant when evaluated from the VHE
> or nVHE code, alternatively checking a static key when called from
> elsewhere in the kernel. On face value, this looks like a case of
> premature optimization, but in fact this allows symbol references on
> VHE-specific code paths to be dropped from the nVHE object.
> 
> Expand the comment in has_vhe() to make this clearer, hopefully
> discouraging anybody from simplifying the code.
> 
> Cc: David Brazdil <dbrazdil@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/include/asm/virt.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 7379f35ae2c6..3218ca17f819 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -111,6 +111,9 @@ static __always_inline bool has_vhe(void)
>  	/*
>  	 * Code only run in VHE/NVHE hyp context can assume VHE is present or
>  	 * absent. Otherwise fall back to caps.
> +	 * This allows the compiler to discard VHE-specific code from the
> +	 * nVHE object, reducing the number of external symbol references
> +	 * needed to link.
>  	 */
>  	if (is_vhe_hyp_code())
>  		return true;
> -- 
> 2.32.0.rc0.204.g9fa02ecfa5-goog
> 