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

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Date:   Sun, 13 Sep 2020 11:43:30 -0400
From:   boris.ostrovsky@...cle.com
To:     Anchal Agarwal <anchalag@...zon.com>, 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



On 8/21/20 6:25 PM, Anchal Agarwal wrote:
> From: Munehisa Kamata <kamatam@...zon.com>  
> 
> Guest hibernation is different from xen suspend/resume/live migration.
> Xen save/restore does not use pm_ops as is needed by guest hibernation.
> Hibernation in guest follows ACPI path and is guest inititated , the
> hibernation image is saved within guest as compared to later modes
> which are xen toolstack assisted and image creation/storage is in
> control of hypervisor/host machine.
> To differentiate between Xen suspend and PM hibernation, keep track
> of the on-going suspend mode by mainly using a new API to keep track of
> SHUTDOWN_SUSPEND state.
> Introduce a simple function that keeps track of on-going suspend mode
> so that PM hibernation code can behave differently according to the
> current suspend mode.
> Since Xen suspend doesn't have corresponding PM event, its main logic
> is modfied to acquire pm_mutex.


lock_system_sleep() is not taking this mutex.


> 
> Though, accquirng pm_mutex is still right thing to do, we may
> see deadlock if PM hibernation is interrupted by Xen suspend.
> PM hibernation depends on xenwatch thread to process xenbus state
> transactions, but the thread will sleep to wait pm_mutex which is
> already held by PM hibernation context in the scenario. Xen shutdown
> code may need some changes to avoid the issue.



Is it Xen's shutdown or suspend code that needs to address this? (Or I
may not understand what the problem is that you are describing)


> 
> +
> +static int xen_pm_notifier(struct notifier_block *notifier,
> +	unsigned long pm_event, void *unused)
> +{
> +	int ret;
> +
> +	switch (pm_event) {
> +	case PM_SUSPEND_PREPARE:
> +	case PM_HIBERNATION_PREPARE:
> +	/* Guest hibernation is not supported for aarch64 currently*/
> +	if (IS_ENABLED(CONFIG_ARM64)) {
> +		ret = NOTIFY_BAD;
> +		break;
> +	}

Indentation.

> +	case PM_RESTORE_PREPARE:
> +	case PM_POST_SUSPEND:
> +	case PM_POST_HIBERNATION:
> +	case PM_POST_RESTORE:
> +	default:
> +		ret = NOTIFY_OK;
> +	}
> +	return ret;
> +};


This whole routine now is

	if (IS_ENABLED(CONFIG_ARM64))
		return NOTIFY_BAD;

	return NOTIFY_OK;

isn't it?


> +
> +static struct notifier_block xen_pm_notifier_block = {
> +	.notifier_call = xen_pm_notifier
> +};
> +
> +static int xen_setup_pm_notifier(void)
> +{
> +	if (!xen_hvm_domain() || xen_initial_domain())
> +		return -ENODEV;


I don't think this works anymore.

In the past your notifier would set suspend_mode (or something) but now
it really doesn't do anything except reports an error in some (ARM) cases.

So I think you should move this check into the notifier.

(And BTW I still think PM_SUSPEND_PREPARE should return an error too.
The fact that we are using "suspend" in xen routine names is irrelevant)



-boris



> +	return register_pm_notifier(&xen_pm_notifier_block);
> +}
> +