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SubjectRe: [PATCH 06/16] arm64: capabilities: Unify the verification
From
Date
On 26/01/18 11:08, Dave Martin wrote:
> On Tue, Jan 23, 2018 at 12:27:59PM +0000, Suzuki K Poulose wrote:
>> Now that each capability describes how to treat the conflicts
>> of CPU cap state vs System wide cap state, we can unify the
>> verification logic to a single place.
>>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>> arch/arm64/kernel/cpufeature.c | 87 ++++++++++++++++++++++++++----------------
>> 1 file changed, 54 insertions(+), 33 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 43c7e992d784..79737034a628 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -1228,6 +1228,54 @@ static void __init enable_cpu_capabilities(const struct arm64_cpu_capabilities *
>> }
>
>>
>> /*
>> + * Run through the list of capabilities to check for conflicts.
>> + * Returns "false" on conflicts.
>> + */
>> +static bool __verify_local_cpu_caps(const struct arm64_cpu_capabilities *caps_list)
>> +{
>> + bool cpu_has_cap, system_has_cap;
>> + const struct arm64_cpu_capabilities *caps = caps_list;
>> +
>> + for (; caps->matches; caps++) {
>> + cpu_has_cap = __this_cpu_has_cap(caps_list, caps->capability);
>
> What's the point of scanning the whole of caps_list? Don't we already
> have the pointer to the right cap struct?
>
> We already know caps->matches is true. Can't we just call
> caps->matches(caps)? That seemed pretty intuitive to me in the old
> code.
>

This was supposed to be fixed by [1] in the "old code". Given we have multiple
entries for a "capability", we could be dealing with the one which doesn't
apply to this CPU and could eventually trigger a wrong conflict below. To
avoid this, we need to make sure use the right values.

>> + system_has_cap = cpus_have_cap(caps->capability);
>> +
>> + if (system_has_cap) {
>> + /*
>> + * Check if the new CPU misses an advertised feature, which is not
>> + * safe to miss.
>> + */
>> + if (!cpu_has_cap && !cpucap_late_cpu_missing_cap_safe(caps))
>> + break;
>> + /*
>> + * We have to issue enable() irrespective of whether the CPU
>> + * has it or not, as it is enabeld system wide. It is upto
>
> enabled
>
>> + * the call back to take appropriate action on this CPU.
>> + */
>> + if (caps->enable)
>> + caps->enable(caps);
>> + } else {
>> + /*
>> + * Check if the CPU has this capability if it isn't safe to
>> + * have when the system doesn't.
>> + */
>
> Possibly most of the commenting here is not needed. The code is pretty
> self-explanatory, so the comments may just be adding clutter.

Sure.

>
> The role of the ->enable() call is the only real subtlety here.
>
>> + if (cpu_has_cap && !cpucap_late_cpu_have_cap_safe(caps))
>> + break;
>> + }
>> + }
>> +
>> + if (caps->matches) {
>> + pr_crit("CPU%d: Detected conflict for capability %d (%s), System: %d, CPU: %d\n",
>> + smp_processor_id(), caps->capability,
>> + caps->desc ? : "no description",
>
> Wouldn't it be a bug for a conflict to occur on a cap with no .desc?
>
> Why can't we just let printk print its default "(null)" for %s
> in this case?

We could.

>
> Alternatively, is there a reason for any cap not to have a description?

Some of them do. e.g, some of them could be "negative" capabilities. e.g,
ARM64_NO_FPSIMD.

>> + system_has_cap, cpu_has_cap);
>> + return false;
>> + }
>> +
>> + return true;
>> +}
>
> Perhaps the capability verification procedure could be made a little
> clearer by splitting this into two functions:
>

As explained above, the code below is not sufficient.


> static bool __verify_local_cpu_cap(const struct arm64_cpu_capabilities *cap)
> {
> bool cpu_has_cap = cap->matches(cap, SCOPE_LOCAL_CPU);
> bool system_has_cap = cpus_have_cap(cap->capability);
>
> if (system_has_cap) {
> if (!cpu_has_cap && !cpucap_late_cpu_missing_cap_safe(cap))
> goto bad;
>
> if (cap->enable)
> /* Enable for this cpu if appropriate: */
> cap->enable(cap);
> } else {
> if (cpu_has_cap && !cpucap_late_cpu_have_cap_safe(cap))
> goto bad;
> }
>
> return true;
>
> bad:
> pr_crit([...]);
> return false;
> }
>
> static bool __verify_local_cpu_caps(const struct arm64_cpu_capabilities *caps)
> {
> while (caps->matches) {
> if (!__verify_local_cpu_cap(caps))
> return false;
>
> ++caps;
> }
>
> return true;
> }


[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-January/552877.html

Cheers
Suzuki

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