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Message-ID: <bd4908d2-cea9-406b-902f-618626e74c88@csgroup.eu>
Date: Wed, 28 Aug 2024 07:46:52 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: 虞陆铭 <luming.yu@...ngroup.cn>
Cc: "shenghui.qu@...ngroup.cn" <shenghui.qu@...ngroup.cn>,
npiggin <npiggin@...il.com>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
mpe <mpe@...erman.id.au>, "luming.yu" <luming.yu@...il.com>,
杨佳龙 <jialong.yang@...ngroup.cn>,
linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier
infrastructure
Hi,
Le 28/08/2024 à 05:17, 虞陆铭 a écrit :
> 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.
As far as I can see, user return notifier infrastructure was implemented
in 2009 for KVM on x86, see
https://lore.kernel.org/all/1253105134-8862-1-git-send-email-avi@redhat.com/
Can you explain what is your usage of that infrastructure with your
patch ? You are talking about debug, what's the added value, what is it
used for ?
Thanks
Christophe
>
> [root@...alhost 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@...roup.eu>;
> Date: Tue, Feb 20, 2024 05:02 PM
> To: "mpe"<mpe@...erman.id.au>; "Aneesh Kumar K.V"<aneesh.kumar@...nel.org>; "虞陆铭"<luming.yu@...ngroup.cn>; "linuxppc-dev"<linuxppc-dev@...ts.ozlabs.org>; "linux-kernel"<linux-kernel@...r.kernel.org>; "npiggin"<npiggin@...il.com>;
> Cc: "shenghui.qu@...ngroup.cn"<shenghui.qu@...ngroup.cn>; "dawei.li@...ngroup.cn"<dawei.li@...ngroup.cn>; "ke.zhao@...ngroup.cn"<ke.zhao@...ngroup.cn>; "luming.yu"<luming.yu@...il.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@...nel.org> writes:
>>>> Luming Yu <luming.yu@...ngroup.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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FKSPP%2Flinux%2Fissues%2F348&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cd9e9b6315413430cfba108dcc7100633%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638604118862628419%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=iIYQTodb9zrGdfmzvnZrIVZ%2BKh2qZjMjT29ddkUpGIw%3D&reserved=0
>
> Correct one is https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Flinuxppc%2Fissues%2Fissues%2F477&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cd9e9b6315413430cfba108dcc7100633%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638604118862637095%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=aJRVdRWu%2F7NQ13jQ6yFLBynXIIfPPPQ3nS4FxiXGNyw%3D&reserved=0
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