linux-kernel - Re: [PATCH] arm64: Run enable method for errata work arounds on late CPUs

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Message-ID: <5d51dd4a-d985-22e5-6583-bcc4bfdf8e6f@arm.com>
Date:   Wed, 17 Jan 2018 14:04:38 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Robin Murphy <robin.murphy@....com>,
        Dave Martin <Dave.Martin@....com>
Cc:     Mark Rutland <mark.rutland@....com>, catalin.marinas@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        Andre Przywara <andre.przywara@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Run enable method for errata work arounds on late
 CPUs

On 17/01/18 13:43, Robin Murphy wrote:
> On 17/01/18 13:31, Suzuki K Poulose wrote:
>> On 17/01/18 13:20, Robin Murphy wrote:
>>> On 17/01/18 12:25, Dave Martin wrote:
>>>> On Wed, Jan 17, 2018 at 10:05:56AM +0000, Suzuki K Poulose wrote:
>>>>> When a CPU is brought up after we have finalised the system
>>>>> wide capabilities (i.e, features and errata), we make sure the
>>>>> new CPU doesn't need a new errata work around which has not been
>>>>> detected already. However we don't run enable() method on the new
>>>>> CPU for the errata work arounds already detected. This could
>>>>> cause the new CPU running without potential work arounds.
>>>>> It is upto the "enable()" method to decide if this CPU should
>>>>> do something about the errata.
>>>>>
>>>>> Fixes: commit 6a6efbb45b7d95c84 ("arm64: Verify CPU errata work arounds on hotplugged CPU")
>>>>> Cc: Will Deacon <will.deacon@....com>
>>>>> Cc: Mark Rutland <mark.rutland@....com>
>>>>> Cc: Andre Przywara <andre.przywara@....com>
>>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>>> Cc: Dave Martin <dave.martin@....com>
>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>>> ---
>>>>>   arch/arm64/kernel/cpu_errata.c | 9 ++++++---
>>>>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
>>>>> index 90a9e465339c..54e41dfe41f6 100644
>>>>> --- a/arch/arm64/kernel/cpu_errata.c
>>>>> +++ b/arch/arm64/kernel/cpu_errata.c
>>>>> @@ -373,15 +373,18 @@ void verify_local_cpu_errata_workarounds(void)
>>>>>   {
>>>>>       const struct arm64_cpu_capabilities *caps = arm64_errata;
>>>>> -    for (; caps->matches; caps++)
>>>>> -        if (!cpus_have_cap(caps->capability) &&
>>>>> -            caps->matches(caps, SCOPE_LOCAL_CPU)) {
>>>>> +    for (; caps->matches; caps++) {
>>>>> +        if (cpus_have_cap(caps->capability)) {
>>>>> +            if (caps->enable)
>>>>> +                caps->enable((void *)caps);
>>>>
>>>> Do we really need this cast?
>>>
>>> Seems to me like the prototype for .enable needs updating. If any existing callback was actually using the (non-const) void* for some purpose (thankfully nothing seems to be), then passing the capability pointer into that would be unlikely to end well anyway.
>>
>> I agree. This was initially written such that we could call it via on_each_cpu().
>> But then we later switched to stop_machine(). And we weren't using the argument until
>> very recently with the introduction of multiple entries for the same capability.
>>
>> I will try to clean this up in a separate series, which would involve cleaning up
>> all the enable(), quite invasive. I would like this to go in for 4.16, as it is>> needed for things like KPTI and some of the existing caps.

Correction, s/KPTI/bp hardening/

> 
> OK, sounds good. For the sake of the immediate fix, perhaps it's cleaner to just pass NULL here if the current callbacks ignore it?

As I said above, we have some users at the moment, so we cant do that.

Suzuki