linux-kernel - Re: [PATCH 1/2] arm64: capabilities: Allow flexibility in scope

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Date:   Thu, 8 Feb 2018 16:31:47 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     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,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] arm64: capabilities: Allow flexibility in scope

On 08/02/18 16:10, Dave Martin wrote:
> On Thu, Feb 08, 2018 at 12:12:37PM +0000, Suzuki K Poulose wrote:
>> So far we have restricted the scopes for the capabilities
>> as follows :
>>   1) Errata workaround check are run all CPUs (i.e, always
>>      SCOPE_LOCAL_CPU)
>>   2) Arm64 features are run only once after the sanitised
>>      feature registers are available using the SCOPE_SYSTEM.
>>
>> This prevents detecting cpu features that can be detected
>> on one or more CPUs with SCOPE_LOCAL_CPU (e.g KPTI). Similarly
>> for errata workaround with SCOPE_SYSTEM.
>>
>> This patch makes sure that we allow flexibility of having
>> any scope for a capability. So, we now run through both
>> arm64_features and arm64_errata in two phases for detection:
>>
>>   a) with SCOPE_LOCAL_CPU filter on each boot time enabled
>>      CPUs.
>>   b) with SCOPE_SYSTEM filter only once after all boot time
>>      enabled CPUs are active.
>>


>>   
>>   static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
>> @@ -1387,13 +1389,15 @@ static void verify_local_cpu_errata_workarounds(void)
>>   static void update_cpu_errata_workarounds(void)
>>   {
>>   	update_cpu_capabilities(arm64_errata,
>> -				ARM64_CPUCAP_SCOPE_ALL,
>> +				ARM64_CPUCAP_SCOPE_LOCAL_CPU,
> 
> In isolation, this looks strange because it seems to handle only a
> subset of arm64_errata now.
> 
> I think I understand the change as follows:
> 
>   * all previously-existing errata workarounds SCOPE_LOCAL_CPU
>     anyway, so the behavior here doesn't change for any existing
>     caps;
> 
>   * the non SCOPE_SYSTEM workarounds (which this patch prepares for)
"the SCOPE_SYSTEM" workarounds..."

>     are handled by the new setup_errata_workaround() path.

> 
> Similarly, the features handling is split into two: one mirroring
> the current behaviour (for SCOPE_SYSTEM this time) and one handling> the rest, for supporting SCOPE_CPU_LOCAL features in subsequent
> patches.

Right. The changes here are :

New behavior:
   - Run SCOPE_LOCAL_CPU filter on arm64_features on all CPUs (newly added with
     this patch) via (newly added)update_cpu_local_features().

Split of existing behavior:
   - Run SCOPE_LOCAL_CPU(instead of the earlier SCOPE_ALL) on all CPUs in
     update_cpu_errata_workarounds()
   - Run SCOPE_SYSTEM filter on arm64_errata, once, via setup_errata_workarounds()

> 
> Does that sound right?
> 
> I'm not sure we need extra comments or documentation; I just want
> to check that I've understood the patch correctly.

So, would you prefer this split to the original patch ?

Cheers
Suzuki