Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()

[Thomas Gleixner]

On Wed, Sep 30 2020 at 20:35, Peter Zijlstra wrote:
> On Wed, Sep 30, 2020 at 08:00:59PM +0200, Thomas Gleixner wrote:
>> 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.
>
> Also, I forgot to add, it accesses ->cpus_mask without the proper
> locking, so it could be reading intermediate state from whatever cpumask
> operation that's in progress.

Yes. I saw that after hitting send. :(

>> 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'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
>> 
>>     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.
>
> That's and, right, not or. because 1) deals with sched_setffinity()
> and 2) deals wit hotplug.

It was meant as AND of course.

> Now 1) requires an arch hook in sched_setaffinity(), something I'm not
> keen on providing, once we provide it, who knows what strange and
> wonderful things archs will dream up.

I don't think so. We can have that magic in core:

#ifdef CONFIG_HAS_PARANOID_L1D_FLUSH
static bool paranoid_l1d_valid(struct task_struct *tsk,
                               const struct cpumask *msk)
{
	if (!test_tsk_thread_flag(tsk, TIF_SPEC_L1D_FLUSH))
        	return true;
        /* Do magic stuff */
        return res;
}
#else
static bool paranoid_l1d_valid(struct task_struct *tsk,
                               const struct cpumask *msk)
{
	return true;
}
#endif

It's a pretty well defined problem and having the magic in core code
prevents an arch hook which allows abuse of all sorts.

And the same applies to enable_l1d_flush_for_task(). The only
architecture specific nonsense are the checks whether the CPU bug is
there and whether the hardware supports L1D flushing.

So we can have:

#ifdef CONFIG_HAS_PARANOID_L1D_FLUSH
int paranoid_l1d_enable(struct task_struct *tsk)
{
        /* Do the SMT validation under the proper locks */
        if (!res)
        	set_task_thread_flag(tsk, TIF_SPEC_L1D_FLUSH);
        return res;
}
#endif

> And 2) also happens on hot-un-plug, when the task's affinity gets
> forced because it became empty. No user feedback there either, and
> information is lost.

Of course. It's both that suddenly SMT gets enabled on a core which was
isolated and when the last isolated core in the tasks CPU mask goes
offline.

> I suppose we can do 2) but send a signal. That would cover all cases and
> keep it in arch code. But yes, that's pretty terrible too.

Bah. I just looked at the condition to flush:

        if (sched_smt_active() && !this_cpu_read(cpu_info.smt_active) &&
                (prev_mm & LAST_USER_MM_L1D_FLUSH))
                l1d_flush_hw();

That fails to flush when SMT is disabled globally. Balbir?

Of course this should be:

        if (!this_cpu_read(cpu_info.smt_active) && (prev_mm & LAST_USER_MM_L1D_FLUSH))
                l1d_flush_hw();

Now we can make this:

        if (unlikely(prev_mm & LAST_USER_MM_L1D_FLUSH)) {
        	if (!this_cpu_read(cpu_info.smt_active))
                	l1d_flush_hw();
                else
                	task_work_add(...);

And that task work clears the flag and sends a signal. We're not going
to send a signal from switch_mm() ....

Thanks,

        tglx