netdev - Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend mode

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Date:   Tue, 15 Sep 2020 15:58:45 -0400
From:   boris.ostrovsky@...cle.com
To:     Anchal Agarwal <anchalag@...zon.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, jgross@...e.com, linux-pm@...r.kernel.org,
        linux-mm@...ck.org, kamatam@...zon.com, sstabellini@...nel.org,
        konrad.wilk@...cle.com, roger.pau@...rix.com, axboe@...nel.dk,
        davem@...emloft.net, rjw@...ysocki.net, len.brown@...el.com,
        pavel@....cz, peterz@...radead.org, eduval@...zon.com,
        sblbir@...zon.com, xen-devel@...ts.xenproject.org,
        vkuznets@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, dwmw@...zon.co.uk,
        benh@...nel.crashing.org
Subject: Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend
 mode



>>
>>
>>>>> +
>>>>> +static int xen_setup_pm_notifier(void)
>>>>> +{
>>>>> +     if (!xen_hvm_domain() || xen_initial_domain())
>>>>> +             return -ENODEV;
>>>>
>>>> I don't think this works anymore.
>>> What do you mean?
>>> The first check is for xen domain types and other is for architecture support.
>>> The reason I put this check here is because I wanted to segregate the two.
>>> I do not want to register this notifier at all for !hmv guest and also if its
>>> an initial control domain.
>>> The arm check only lands in notifier because once hibernate() api is called ->
>>> calls pm_notifier_call_chain for PM_HIBERNATION_PREPARE this will fail for
>>> aarch64.
>>> Once we have support for aarch64 this notifier can go away altogether.
>>>
>>> Is there any other reason I may be missing why we should move this check to
>>> notifier?
>>
>>
>> Not registering this notifier is equivalent to having it return NOTIFY_OK.
>>
> How is that different from current behavior?
>>
>> In your earlier versions just returning NOTIFY_OK was not sufficient for
>> hibernation to proceed since the notifier would also need to set
>> suspend_mode appropriately. But now your notifier essentially filters
>> out unsupported configurations. And so if it is not called your
>> configuration (e.g. PV domain) will be considered supported.
>>
> I am sorry if I am having a bit of hard time understanding this. 
> How will it be considered supported when its not even registered? My
> understanding is if its not registered, it will not land in notifier call chain
> which is invoked in pm_notifier_call_chain().


Returning an error from xen_setup_pm_notifier() doesn't have any effect
on whether hibernation will start. It's the notifier that can stop it.

> 
> As Roger, mentioned in last series none of this should be a part of PVH dom0 
> hibernation as its not tested but this series should also not break anything.
> If I register this notifier for PVH dom0 and return error later that will alter
> the current behavior right?
> 
> If a pm_notifier for pvh dom0 is not registered then it will not land in the
> notifier call chain and system will work as before this series.
> If I look for unsupported configurations, then !hvm domain is also one but we
> filter that out at the beginning and don't even bother about it.
> 
> Unless you mean guest running VMs itself? [Trying to read between the lines may
> not be the case though]



In hibernate():

        error = __pm_notifier_call_chain(PM_HIBERNATION_PREPARE, -1,
&nr_calls);
        if (error) {
                nr_calls--;
                goto Exit;
        }


Is you don't have notifier registered (as will be the case with PV
domains and dom0) you won't get an error and proceed with hibernation.
(And now I actually suspect it didn't work even with your previous patches)


But something like this I think will do what you want:


static int xen_pm_notifier(struct notifier_block *notifier,
	unsigned long pm_event, void *unused)

{

       if (IS_ENABLED(CONFIG_ARM64) ||
	   !xen_hvm_domain() || xen_initial_domain() ||
	   (pm_event == PM_SUSPEND_PREPARE)) {
		if ((pm_event == PM_SUSPEND_PREPARE) || (pm_event ==
PM_HIBERNATION_PREPARE))
			pr_warn("%s is not supported for this guest",
				(pm_event == PM_SUSPEND_PREPARE) ?
				"Suspend" : "Hibernation");
                return NOTIFY_BAD;

        return NOTIFY_OK;

}

static int xen_setup_pm_notifier(void)
{
	return register_pm_notifier(&xen_pm_notifier_block);
}


I tried to see if there is a way to prevent hibernation without using
notifiers (like having a global flag or something) but didn't find
anything obvious. Perhaps others can point to a simpler way of doing this.


-boris