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SubjectRe: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()
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On 1/10/20 4:00 am, Thomas Gleixner wrote:
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>
>
>
> On Wed, Sep 30 2020 at 19:03, Peter Zijlstra wrote:
>> On Wed, Sep 30, 2020 at 05:40:08PM +0200, Thomas Gleixner wrote:
>> Also, that preempt_disable() in there doesn't actually do anything.
>> Worse, preempt_disable(); for_each_cpu(); is an anti-pattern. It mixes
>> static_cpu_has() and boot_cpu_has() in the same bloody condition and has
>> a pointless ret variable.
>

I was being a bit crazy in mixing the two, considering that there might
be CPUs that do not support L1D flush (others might in the same system,
which is insane)

> I absolutely agree and I really missed it when looking at it before
> merging. cpus_read_lock()/unlock() is the right thing to do if at all.
>

It seems like the right thing to do, get_cpu() used to be the old method.
The idea is to use cpu_data(i) in a hotplug safe manner.

>> It's shoddy code, that only works if you align the planets right. We
>> really shouldn't provide interfaces that are this bad.
>>
>> It's correct operation is only by accident.



>
> True :(
>
> I understand Balbirs problem and it makes some sense to provide a
> solution. We can:
>
> 1) reject set_affinity() if the task has that flush muck enabled
> and user space tries to move it to a SMT enabled core

I thought of this and it would be difficult to debug for users, taskset -c
would not work on applications that flush, etc, etc.

>
> 2) disable the muck if if detects that it is runs on a SMT enabled
> core suddenly (hotplug says hello)
>
> This one is nasty because there is no feedback to user space
> about the wreckage.

Yes, agreed.

>

Trying to look at the concerns, I wonder if this can still be saved

- if (!boot_cpu_has_bug(X86_BUG_L1TF) ||
- !static_cpu_has(X86_FEATURE_FLUSH_L1D))
+ if (!static_cpu_has(X86_BUG_L1TF) ||
+ !static_cpu_has(X86_FEATURE_FLUSH_L1D))
return -EINVAL;

- cpu = get_cpu();

+ cpus_read_lock();
for_each_cpu(i, &tsk->cpus_mask) {
if (cpu_data(i).smt_active == true) {
- put_cpu();
+ cpus_read_unlock();
return -EINVAL;
}
}
+ cpus_read_unlock();

set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
- put_cpu();
return ret;
}

I don't like the idea of iterating CPUs in the cpumask to check if they
all have SMT disabled, but that is a requirement for flush

Balbir

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