Re: [PATCH v2 04/17] arm64: Add KVM_HVHE capability and has_hvhe() predicate

[Oliver Upton <oliver.upton@linux.dev>]

Hey Marc,

I'm an idiot and was responding to v1. Here's the same damn comment, but
on v2!

On Fri, May 26, 2023 at 03:33:35PM +0100, Marc Zyngier wrote:
> Expose a capability keying the hVHE feature as well as a new
> predicate testing it. Nothing is so far using it, and nothing
> is enabling it yet.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h |  1 +
>  arch/arm64/include/asm/virt.h       |  8 ++++++++
>  arch/arm64/kernel/cpufeature.c      | 15 +++++++++++++++
>  arch/arm64/tools/cpucaps            |  1 +
>  4 files changed, 25 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index bc1009890180..3d4b547ae312 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -16,6 +16,7 @@
>  #define cpu_feature(x)		KERNEL_HWCAP_ ## x
>  
>  #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR	0
> +#define ARM64_SW_FEATURE_OVERRIDE_HVHE		4
>  
>  #ifndef __ASSEMBLY__
>  
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 91029709d133..5f84a87a6a2d 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -145,6 +145,14 @@ static __always_inline bool is_protected_kvm_enabled(void)
>  		return cpus_have_final_cap(ARM64_KVM_PROTECTED_MODE);
>  }
>  
> +static __always_inline bool has_hvhe(void)
> +{
> +	if (is_vhe_hyp_code())
> +		return false;
> +
> +	return cpus_have_final_cap(ARM64_KVM_HVHE);
> +}
> +
>  static inline bool is_hyp_nvhe(void)
>  {
>  	return is_hyp_mode_available() && !is_kernel_in_hyp_mode();
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 2d2b7bb5fa0c..04ef60571b37 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1998,6 +1998,15 @@ static bool has_nested_virt_support(const struct arm64_cpu_capabilities *cap,
>  	return true;
>  }
>  
> +static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> +			  int __unused)
> +{
> +	u64 val;
> +
> +	val = arm64_sw_feature_override.val & arm64_sw_feature_override.mask;
> +	return cpuid_feature_extract_unsigned_field(val, ARM64_SW_FEATURE_OVERRIDE_HVHE);
> +}

Does this need to test ID_AA64MMFR1_EL1.VH as well? Otherwise I don't
see what would stop us from attempting hVHE on a system with asymmetric
support for VHE, as the software override was only evaluated on the boot
CPU.

>  #ifdef CONFIG_ARM64_PAN
>  static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
>  {
> @@ -2643,6 +2652,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.cpu_enable = cpu_enable_dit,
>  		ARM64_CPUID_FIELDS(ID_AA64PFR0_EL1, DIT, IMP)
>  	},
> +	{
> +		.desc = "VHE for hypervisor only",
> +		.capability = ARM64_KVM_HVHE,
> +		.type = ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE,
> +		.matches = hvhe_possible,
> +	},
>  	{},
>  };
>  
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index 40ba95472594..3c23a55d7c2f 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -47,6 +47,7 @@ HAS_TLB_RANGE
>  HAS_VIRT_HOST_EXTN
>  HAS_WFXT
>  HW_DBM
> +KVM_HVHE
>  KVM_PROTECTED_MODE
>  MISMATCHED_CACHE_TYPE
>  MTE
> -- 
> 2.34.1
> 

-- 
Thanks,
Oliver

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