Re: [PATCH v4 04/12] KVM: arm64: Add ARM64_HAS_LPA2 CPU capability

[Marc Zyngier <maz@kernel.org>]

On Mon, 09 Oct 2023 19:50:00 +0100,
Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
> Expose FEAT_LPA2 as a capability so that we can take advantage of
> alternatives patching in both the kernel and hypervisor.
> 
> Although FEAT_LPA2 presence is advertised separately for stage1 and
> stage2, the expectation is that in practice both stages will either
> support or not support it. Therefore, for the case where KVM is present,
> we combine both into a single capability, allowing us to simplify the
> implementation. For the case where KVM is not present, we only care
> about stage1.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  arch/arm64/include/asm/cpufeature.h |  5 ++++
>  arch/arm64/kernel/cpufeature.c      | 46 +++++++++++++++++++++++++++++
>  arch/arm64/tools/cpucaps            |  1 +
>  3 files changed, 52 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 5bba39376055..b1292ec88538 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -831,6 +831,11 @@ static inline bool system_supports_tlb_range(void)
>  		cpus_have_const_cap(ARM64_HAS_TLB_RANGE);
>  }
>  
> +static inline bool system_supports_lpa2(void)
> +{
> +	return cpus_have_const_cap(ARM64_HAS_LPA2);

cpus_have_const_cap() is going away. You may want to look at Mark's
series to see how to replace this one.

> +}
> +
>  int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);
>  bool try_emulate_mrs(struct pt_regs *regs, u32 isn);
>  
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 444a73c2e638..1ccb1fe0e310 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1746,6 +1746,46 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  	return !meltdown_safe;
>  }
>  
> +static inline bool has_lpa2_at_stage1(u64 mmfr0)

Why inline? It isn't like this has any performance implication...

> +{
> +#if defined(CONFIG_ARM64_4K_PAGES) || defined(CONFIG_ARM64_16K_PAGES)
> +	unsigned int tgran;
> +
> +	tgran = cpuid_feature_extract_unsigned_field(mmfr0,
> +						ID_AA64MMFR0_EL1_TGRAN_SHIFT);
> +	return tgran == ID_AA64MMFR0_EL1_TGRAN_LPA2;
> +#else
> +	return false;
> +#endif

Writing this using IS_ENABLED() would be slightly more pleasing to my
tired eyes... ;-)

> +}
> +
> +static inline bool has_lpa2_at_stage2(u64 mmfr0)
> +{
> +#if defined(CONFIG_ARM64_4K_PAGES) || defined(CONFIG_ARM64_16K_PAGES)
> +	unsigned int tgran;
> +
> +	tgran = cpuid_feature_extract_unsigned_field(mmfr0,
> +						ID_AA64MMFR0_EL1_TGRAN_2_SHIFT);
> +	return tgran == ID_AA64MMFR0_EL1_TGRAN_2_SUPPORTED_LPA2;
> +#else
> +	return false;
> +#endif
> +}
> +
> +static bool has_lpa2(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	u64 mmfr0;
> +	bool ret;
> +
> +	mmfr0 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1);
> +	ret = has_lpa2_at_stage1(mmfr0);
> +
> +	if (kvm_get_mode() != KVM_MODE_NONE)
> +		ret = ret && has_lpa2_at_stage2(mmfr0);

Isn't it too late to go back on the decision to use LPA2 at S1 if you
realise that S2 doesn't support it?

> +
> +	return ret;
> +}
> +
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>  #define KPTI_NG_TEMP_VA		(-(1UL << PMD_SHIFT))
>  
> @@ -2719,6 +2759,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.matches = has_cpuid_feature,
>  		ARM64_CPUID_FIELDS(ID_AA64MMFR2_EL1, EVT, IMP)
>  	},
> +	{
> +		.desc = "Large Physical Address 2",
> +		.capability = ARM64_HAS_LPA2,
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.matches = has_lpa2,
> +	},
>  	{},
>  };
>  
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index dea3dc89234b..07f3957b8488 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -36,6 +36,7 @@ HAS_GIC_PRIO_MASKING
>  HAS_GIC_PRIO_RELAXED_SYNC
>  HAS_HCX
>  HAS_LDAPR
> +HAS_LPA2
>  HAS_LSE_ATOMICS
>  HAS_MOPS
>  HAS_NESTED_VIRT

Why isn't this patch the first or second in the series? You could use
it to drive the LPA2 decision in the patch #2, avoiding the ugly lpa2
flag...

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.