linux-kernel - Re: [PATCH v3 15/22] arm64: capabilities: Change scope of VHE to Boot CPU feature

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Message-ID: <25d774db-0ebd-b06d-20b5-5e31ede58fa3@arm.com>
Date:   Thu, 8 Mar 2018 12:10:22 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com,
        robin.murphy@....com
Subject: Re: [PATCH v3 15/22] arm64: capabilities: Change scope of VHE to Boot
 CPU feature

On 12/02/18 17:17, Dave Martin wrote:
> On Fri, Feb 09, 2018 at 05:54:59PM +0000, Suzuki K Poulose wrote:
>> We expect all CPUs to be running at the same EL inside the kernel
>> with or without VHE enabled and we have strict checks to ensure
>> that any mismatch triggers a kernel panic. If VHE is enabled,
>> we use the feature based on the boot CPU and all other CPUs
>> should follow. This makes it a perfect candidate for a cpability
> 
> capability
> 
>> based on the boot CPU,  which should be matched by all the CPUs
>> (both when is ON and OFF). This saves us some not-so-pretty
>> hooks and special code, just for verifying the conflict.
>>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Dave Martin <dave.martin@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   arch/arm64/include/asm/cpufeature.h |  7 +++++++
>>   arch/arm64/include/asm/virt.h       |  6 ------
>>   arch/arm64/kernel/cpufeature.c      |  5 +++--
>>   arch/arm64/kernel/smp.c             | 38 -------------------------------------
>>   4 files changed, 10 insertions(+), 46 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index 5f56a8342065..dfce93f79ae7 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -276,6 +276,13 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>>   	(ARM64_CPUCAP_SCOPE_LOCAL_CPU		|	\
>>   	 ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU)
>>   
>> +/*
>> + * Critical CPU feature used early in the boot based on the boot CPU.
>> + * The feature should be matched by all booting CPU (both miss and hit
>> + * cases).
>> + */
>> +#define ARM64_CPUCAP_CRITICAL_BOOT_CPU_FEATURE ARM64_CPUCAP_SCOPE_BOOT_CPU
>> +
> 
> Nit: would it be consistent with the uses we already have for the word
> "strict" to use that word here?  i.e.,
> ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE.
> Or do you think that would be more confusing?

We don't use the "STRICT" tag anymore. Moreover, I used CRITICAL to indicate
that it is special in a way that all the "late" CPUs (in this case all
secondaries) should match the "state" of the capability  (i.e, both ON and OFF)
as that of the boot CPU. I am OK to change it to STRICT.

> 
> Otherwise, "critical" sounds a bit like we depend on the capability
> being available.
> 
> If "strict" doesn't fit though and no other option suggests itself,
> it's probably not worth changing this.
> 
>>   struct arm64_cpu_capabilities {
>>   	const char *desc;
>>   	u16 capability;
>> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
>> index c5f89442785c..9d1e24e030b3 100644
>> --- a/arch/arm64/include/asm/virt.h
>> +++ b/arch/arm64/include/asm/virt.h
>> @@ -102,12 +102,6 @@ static inline bool has_vhe(void)
>>   	return false;
>>   }
>>   
>> -#ifdef CONFIG_ARM64_VHE
>> -extern void verify_cpu_run_el(void);
>> -#else
>> -static inline void verify_cpu_run_el(void) {}
>> -#endif
>> -
>>   #endif /* __ASSEMBLY__ */
>>   
>>   #endif /* ! __ASM__VIRT_H */
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 7625e2962e2b..f66e66c79916 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -1016,11 +1016,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>>   	},
>>   #endif /* CONFIG_ARM64_PAN */
>>   	{
>> +#ifdef CONFIG_ARM64_VHE
>>   		.desc = "Virtualization Host Extensions",
>>   		.capability = ARM64_HAS_VIRT_HOST_EXTN,
>> -		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
>> +		.type = ARM64_CPUCAP_CRITICAL_BOOT_CPU_FEATURE,
>>   		.matches = runs_at_el2,
>>   		.cpu_enable = cpu_copy_el2regs,
>> +#endif
> 
> Shouldn't the #ifdef...#endif be outside the { ... },?
> 
> Otherwise this yields an empty block that will truncate the list in the
> CONFIG_ARM64_VHE case...

Good catch. You're right, I will fix it.

> 
> 
> Removal of this block for !CONFIG_ARM64_VHE is a change rather than just
> refactoring, so the commit message should explain it.

Ok.

Cheers
Suzuki