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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 05/17] arm64: Don't enable VHE for the kernel if
 OVERRIDE_HVHE is set
Message-ID: <ZHhJmJU/m//uTI9n@linux.dev>
References: <20230526143348.4072074-1-maz@kernel.org>
 <20230526143348.4072074-6-maz@kernel.org>
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In-Reply-To: <20230526143348.4072074-6-maz@kernel.org>
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On Fri, May 26, 2023 at 03:33:36PM +0100, Marc Zyngier wrote:
> If the OVERRIDE_HVHE SW override is set (as a precursor of
> the KVM_HVHE capability), do not enable VHE for the kernel
> and drop to EL1 as if VHE was either disabled or unavailable.
> 
> Further changes will enable VHE at EL2 only, with the kernel
> still running at EL1.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kernel/hyp-stub.S | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> index 9439240c3fcf..5c71e1019545 100644
> --- a/arch/arm64/kernel/hyp-stub.S
> +++ b/arch/arm64/kernel/hyp-stub.S
> @@ -82,7 +82,15 @@ SYM_CODE_START_LOCAL(__finalise_el2)
>  	tbnz	x1, #0, 1f
>  
>  	// Needs to be VHE capable, obviously
> -	check_override id_aa64mmfr1 ID_AA64MMFR1_EL1_VH_SHIFT 2f 1f x1 x2
> +	check_override id_aa64mmfr1 ID_AA64MMFR1_EL1_VH_SHIFT 0f 1f x1 x2
> +
> +0:	// Check whether we only want the hypervisor to run VHE, not the kernel
> +	adr_l	x1, arm64_sw_feature_override
> +	ldr	x2, [x1, FTR_OVR_VAL_OFFSET]
> +	ldr	x1, [x1, FTR_OVR_MASK_OFFSET]
> +	and	x2, x2, x1

nit: is applying the mask even necessary? I get it in the context of an
overlay on top of an ID register, but the software features are more of
a synthetic ID register in their own right.

Same nit applies to the kaslr case as well.

> +	ubfx	x2, x2, #ARM64_SW_FEATURE_OVERRIDE_HVHE, #4
> +	cbz	x2, 2f
>  
>  1:	mov_q	x0, HVC_STUB_ERR
>  	eret
> -- 
> 2.34.1
> 

-- 
Thanks,
Oliver