Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier infrastructure

[虞陆铭 <luming.yu@shingroup.cn>]

Hi,

it appears the little feature might require a little bit more work to find its value of the patch.

Using the following debug module ,  some debugging shows the TIF_USER_RETURN_NOTIFY
bit is propagated in __switch_to among tasks , but USER_RETURN_NOTIFY call back seems to 
be dropped somewhere on somone who carries the bit return to user space.
side notes:
there is an issue that the module symbols is not appended to /sys/kernel/debug/tracing/available_filter_functions
which should be sovled first to make it easier for further debuggig. 

[root@localhost linux]# cat lib/user-return-test.c
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/container_of.h>
#include <linux/user-return-notifier.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/sched.h>

MODULE_LICENSE("GPL");


struct test_user_return {
        struct user_return_notifier urn;
        bool registered;
        int urn_value_changed;
        struct task_struct *worker;
};

static struct test_user_return __percpu *user_return_test;

static void test_user_return_cb(struct user_return_notifier *urn)
{
        struct test_user_return *tur =
                container_of(urn, struct test_user_return, urn);
        unsigned long flags;

        local_irq_save(flags);
        tur->urn_value_changed++;
        local_irq_restore(flags);
        return;
}

static int test_user_return_worker(void *tur)
{
        struct test_user_return *t;
        t = (struct test_user_return *) tur;
        preempt_disable();
        user_return_notifier_register(&t->urn);
        preempt_enable();
        t->registered = true;
        while (!kthread_should_stop()) {
                static int err_rate = 0;

                msleep (1000);
                if (!test_thread_flag(TIF_USER_RETURN_NOTIFY) && (err_rate == 0)) {
                        pr_err("TIF_USER_RETURN_NOTIFY is lost");
                        err_rate++;
                }
        }
        return 0;
}
static int init_test_user_return(void)
{
        int r = 0;

        user_return_test = alloc_percpu(struct test_user_return);
        if (!user_return_test) {
                pr_err("failed to allocate percpu test_user_return\n");
                r = -ENOMEM;
                goto exit;
        }
        {
                unsigned int cpu;
                struct task_struct *task;
                struct test_user_return *tur;

                for_each_online_cpu(cpu) {
                        tur = per_cpu_ptr(user_return_test, cpu);
                        if (!tur->registered) {
                                tur->urn.on_user_return = test_user_return_cb;
                                task = kthread_create(test_user_return_worker,
                                        tur, "test_user_return");
                                if (IS_ERR(task))
                                        pr_err("no test_user_return kthread created for cpu %d",cpu);
                                else {
                                        tur->worker = task;
                                        wake_up_process(task);
                                }
                        }
                }
        }

exit:
        return r;
}
static void exit_test_user_return(void)
{
        struct test_user_return *tur;
        int i,ret=0;

        for_each_online_cpu(i) {
                tur = per_cpu_ptr(user_return_test, i);
                if (tur->registered) {
                        pr_info("[cpu=%d, %d] ", i, tur->urn_value_changed);
                        user_return_notifier_unregister(&tur->urn);
                        tur->registered = false;
                }
                if (tur->worker) {
                        ret = kthread_stop(tur->worker);
                        if (ret)
                                pr_err("can't stop test_user_return kthread for cpu %d", i);
                }
        }
        free_percpu(user_return_test);
        return;
}

module_init(init_test_user_return);
module_exit(exit_test_user_return);

 
------------------ Original ------------------
From:  "Christophe Leroy"<christophe.leroy@csgroup.eu>;
Date:  Tue, Feb 20, 2024 05:02 PM
To:  "mpe"<mpe@ellerman.id.au>; "Aneesh Kumar K.V"<aneesh.kumar@kernel.org>; "虞陆铭"<luming.yu@shingroup.cn>; "linuxppc-dev"<linuxppc-dev@lists.ozlabs.org>; "linux-kernel"<linux-kernel@vger.kernel.org>; "npiggin"<npiggin@gmail.com>; 
Cc:  "shenghui.qu@shingroup.cn"<shenghui.qu@shingroup.cn>; "dawei.li@shingroup.cn"<dawei.li@shingroup.cn>; "ke.zhao@shingroup.cn"<ke.zhao@shingroup.cn>; "luming.yu"<luming.yu@gmail.com>; 
Subject:  Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier infrastructure

 



Le 20/02/2024 à 09:51, Christophe Leroy a écrit :
> 
> 
> Le 19/12/2023 à 07:33, Michael Ellerman a écrit :
>> Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:
>>> Luming Yu <luming.yu@shingroup.cn> writes:
>>>
>>>> Before we have powerpc to use the generic entry infrastructure,
>>>> the call to fire user return notifier is made temporarily in powerpc
>>>> entry code.
>>>>
>>>
>>> It is still not clear what will be registered as user return notifier.
>>> Can you summarize that here?
>>
>> fire_user_return_notifiers() is defined in kernel/user-return-notifier.c
>>
>> That's built when CONFIG_USER_RETURN_NOTIFIER=y.
>>
>> That is not user selectable, it's only enabled by:
>>
>> arch/x86/kvm/Kconfig:        select USER_RETURN_NOTIFIER
>>
>> So it looks to me like (currently) it's always a nop and does nothing.
>>
>> Which makes me wonder what the point of wiring this feature up is :)
>> Maybe it's needed for some other feature I don't know about?
>>
>> Arguably we could just enable it because we can, and it currently does
>> nothing so it's unlikely to break anything. But that also makes it
>> impossible to test the implementation is correct, and runs the risk that
>> one day in the future when it does get enabled only then do we discover
>> it doesn't work.
> 
> Opened an "issue" for the day we need it:
> https://github.com/KSPP/linux/issues/348

Correct one is https://github.com/linuxppc/issues/issues/477