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

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Date:   Wed, 17 Jan 2018 13:31:50 +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 13:20, 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.

Cheers
Suzuki