linux-kernel - Re: [PATCH v2 3/3] arm64: cpufeature: Change 32-bit EL0 to display enabled cores

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Date:   Thu, 19 Oct 2023 11:14:29 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jeremy Linton <jeremy.linton@....com>,
        Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        will@...nel.org, mark.rutland@....com, anshuman.khandual@....com,
        krisman@...e.de, broonie@...nel.org, james.morse@....com,
        ionela.voinescu@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] arm64: cpufeature: Change 32-bit EL0 to display
 enabled cores

On 2023-10-18 16:42, Jeremy Linton wrote:
> On 10/18/23 07:43, Marc Zyngier wrote:
>> On Tue, 17 Oct 2023 20:15:43 +0100,
>> Jeremy Linton <jeremy.linton@....com> wrote:
>>>
>>> Hi,
>>>
>>> On 10/17/23 13:01, Marc Zyngier wrote:
>>>> On Tue, 17 Oct 2023 06:23:22 +0100,
>>>> Jeremy Linton <jeremy.linton@....com> wrote:
>>>>>
>>>>> Now that we have the ability to display the list of cores
>>>>> with a feature when it is selectivly enabled, lets display the
>>>>> cores enabled for 32-bit use at EL0.
>>>>>
>>>>> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
>>>>> ---
>>>>>    arch/arm64/kernel/cpufeature.c | 15 +++++++++++++--
>>>>>    1 file changed, 13 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/kernel/cpufeature.c 
>>>>> b/arch/arm64/kernel/cpufeature.c
>>>>> index b7b67bac0e60..512cbe446b41 100644
>>>>> --- a/arch/arm64/kernel/cpufeature.c
>>>>> +++ b/arch/arm64/kernel/cpufeature.c
>>>>> @@ -1533,8 +1533,17 @@ static bool has_32bit_el0(const struct 
>>>>> arm64_cpu_capabilities *entry, int scope)
>>>>>        if (!has_cpuid_feature(entry, scope))
>>>>>            return allow_mismatched_32bit_el0;
>>>>>    -    if (scope == SCOPE_SYSTEM)
>>>>> -        pr_info("detected: 32-bit EL0 Support\n");
>>>>> +    if (scope == SCOPE_SYSTEM) {
>>>>> +        struct arm64_cpu_capabilities *has_32bit;
>>>>> +
>>>>> +        has_32bit = (struct arm64_cpu_capabilities *)entry;
>>>>> +
>>>>> +        has_32bit->cpus = system_32bit_el0_cpumask();
>>>>
>>>> This seems rather dodgy. 'entry' comes from a static const array which
>>>> will, in all likelihood be mapped R/O pretty soon after the initial
>>>> CPU bringup. Try offlining/onlining a CPU and you should see a
>>>> firework similar to what I have below (I hacked the CnP property, but
>>>> that's no different from what you are doing):
>>>
>>> Yes, dodgy is a good word. The other choices, maintain a mask just for
>>> the print or dump the static key and always use the cpu_32bit_el0_mask
>>> or some combination, weren't much better in the "ick" category. If
>>> anyone sees a better way I'm open to suggestion, although simply
>>> dropping this last patch is fine too.
>>
>> An obvious choice would be to replace the 'cpus' cpumask with a
>> function that evaluates a cpumask stored separately.
> 
> Right, I was too busy trying to cleanup the 32-bit mask, that works too.

But if a cpumask does already exist, wouldn't the *most* logical thing 
be to use it? If it is somehow harmful to the existing machinery to set 
all of the CPUs in cpu_32bit_el0_mask if all of the CPUs support 32-bit 
EL0, that seems like a mind-bogglingly dumb design which deserves fixing 
in its own right.

To me the obvious thing would be to allocate and populate 
cpu_32bit_el0_mask unconditionally so the initial feature detection can 
work as normal, and then if anyone's really really bothered, free it 
again later if we determine there's no mismatch.

Thanks,
Robin.