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Date:   Tue, 4 Oct 2022 16:24:54 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
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Subject: RE: [PATCH V3 10/11] drivers/hv/vmbus, video/hyperv_fb: Untangle and
 refactor Hyper-V panic notifiers

From: Guilherme G. Piccoli <gpiccoli@...lia.com> Sent: Friday, August 19, 2022 3:18 PM
> 
> Currently Hyper-V guests are among the most relevant users of the panic
> infrastructure, like panic notifiers, kmsg dumpers, etc. The reasons rely
> both in cleaning-up procedures (closing hypervisor <-> guest connection,
> disabling some paravirtualized timer) as well as to data collection
> (sending panic information to the hypervisor) and framebuffer management.
> 
> The thing is: some notifiers are related to others, ordering matters, some
> functionalities are duplicated and there are lots of conditionals behind
> sending panic information to the hypervisor. As part of an effort to
> clean-up the panic notifiers mechanism and better document things, we
> hereby address some of the issues/complexities of Hyper-V panic handling
> through the following changes:
> 
> (a) We have die and panic notifiers on vmbus_drv.c and both have goals of
> sending panic information to the hypervisor, though the panic notifier is
> also responsible for a cleaning-up procedure.
> 
> This commit clears the code by splitting the panic notifier in two, one
> for closing the vmbus connection whereas the other is only for sending
> panic info to hypervisor. With that, it was possible to merge the die and
> panic notifiers in a single/well-documented function, and clear some
> conditional complexities on sending such information to the hypervisor.
> 
> (b) There is a Hyper-V framebuffer panic notifier, which relies in doing
> a vmbus operation that demands a valid connection. So, we must order this
> notifier with the panic notifier from vmbus_drv.c, to guarantee that the
> framebuffer code executes before the vmbus connection is unloaded.
> 
> Also, this commit removes a useless header.
> 
> Although there is code rework and re-ordering, we expect that this change
> has no functional regressions but instead optimize the path and increase
> panic reliability on Hyper-V. This was tested on Hyper-V with success.
> 
> Cc: Andrea Parri (Microsoft) <parri.andrea@...il.com>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> Cc: Wei Liu <wei.liu@...nel.org>
> Reviewed-by: Michael Kelley <mikelley@...rosoft.com>
> Tested-by: Fabio A M Martins <fabiomirmar@...il.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
> 
> ---
> 
> V3:
> - Added Michael's review tag - thanks!
> 
> V2:
> - Unfortunately we cannot rely in the crash shutdown (custom) handler
> to perform the vmbus unload - arm64 architecture doesn't have this
> "feature" [0]. So, in V2 we kept the notifier behavior and always
> unload the vmbus connection, no matter what - thanks Michael for
> pointing that;
> 
> - Removed the Fixes tags as per Michael suggestion;
> 
> - As per Petr suggestion, we abandoned the idea of distinguish among
> notifiers using an id - so, in V2 we rely in the old and good address
> comparison for that. Thanks Petr for the enriching discussion!
> 
> [0] https://lore.kernel.org/lkml/427a8277-49f0-4317-d6c3-4a15d7070e55@igalia.com/
> 
> 
>  drivers/hv/vmbus_drv.c          | 109 +++++++++++++++++++-------------
>  drivers/video/fbdev/hyperv_fb.c |   8 +++
>  2 files changed, 74 insertions(+), 43 deletions(-)
> 

Tested this patch in combination with Patch 9 in this series.  Verified
that both the panic and die paths work correctly with notification to
Hyper-V via hyperv_report_panic() or via hv_kmsg_dump().  Hyper-V
framebuffer is updated as expected, though I did not reproduce
a case where the ring buffer lock is held.  vmbus_initiate_unload() runs
as expected.

Tested-by: Michael Kelley <mikelley@...rosoft.com>

> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 23c680d1a0f5..0a77e2bb0b70 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -25,7 +25,6 @@
>  #include <linux/sched/task_stack.h>
> 
>  #include <linux/delay.h>
> -#include <linux/notifier.h>
>  #include <linux/panic_notifier.h>
>  #include <linux/ptrace.h>
>  #include <linux/screen_info.h>
> @@ -69,53 +68,74 @@ static int hyperv_report_reg(void)
>  	return !sysctl_record_panic_msg || !hv_panic_page;
>  }
> 
> -static int hyperv_panic_event(struct notifier_block *nb, unsigned long val,
> +/*
> + * The panic notifier below is responsible solely for unloading the
> + * vmbus connection, which is necessary in a panic event.
> + *
> + * Notice an intrincate relation of this notifier with Hyper-V
> + * framebuffer panic notifier exists - we need vmbus connection alive
> + * there in order to succeed, so we need to order both with each other
> + * [see hvfb_on_panic()] - this is done using notifiers' priorities.
> + */
> +static int hv_panic_vmbus_unload(struct notifier_block *nb, unsigned long val,
>  			      void *args)
> +{
> +	vmbus_initiate_unload(true);
> +	return NOTIFY_DONE;
> +}
> +static struct notifier_block hyperv_panic_vmbus_unload_block = {
> +	.notifier_call	= hv_panic_vmbus_unload,
> +	.priority	= INT_MIN + 1, /* almost the latest one to execute */
> +};
> +
> +static int hv_die_panic_notify_crash(struct notifier_block *self,
> +				     unsigned long val, void *args);
> +
> +static struct notifier_block hyperv_die_report_block = {
> +	.notifier_call = hv_die_panic_notify_crash,
> +};
> +static struct notifier_block hyperv_panic_report_block = {
> +	.notifier_call = hv_die_panic_notify_crash,
> +};
> +
> +/*
> + * The following callback works both as die and panic notifier; its
> + * goal is to provide panic information to the hypervisor unless the
> + * kmsg dumper is used [see hv_kmsg_dump()], which provides more
> + * information but isn't always available.
> + *
> + * Notice that both the panic/die report notifiers are registered only
> + * if we have the capability HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE set.
> + */
> +static int hv_die_panic_notify_crash(struct notifier_block *self,
> +				     unsigned long val, void *args)
>  {
>  	struct pt_regs *regs;
> +	bool is_die;
> 
> -	vmbus_initiate_unload(true);
> -
> -	/*
> -	 * Hyper-V should be notified only once about a panic.  If we will be
> -	 * doing hv_kmsg_dump() with kmsg data later, don't do the notification
> -	 * here.
> -	 */
> -	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE
> -	    && hyperv_report_reg()) {
> +	/* Don't notify Hyper-V unless we have a die oops event or panic. */
> +	if (self == &hyperv_panic_report_block) {
> +		is_die = false;
>  		regs = current_pt_regs();
> -		hyperv_report_panic(regs, val, false);
> +	} else { /* die event */
> +		if (val != DIE_OOPS)
> +			return NOTIFY_DONE;
> +
> +		is_die = true;
> +		regs = ((struct die_args *)args)->regs;
>  	}
> -	return NOTIFY_DONE;
> -}
> -
> -static int hyperv_die_event(struct notifier_block *nb, unsigned long val,
> -			    void *args)
> -{
> -	struct die_args *die = args;
> -	struct pt_regs *regs = die->regs;
> -
> -	/* Don't notify Hyper-V if the die event is other than oops */
> -	if (val != DIE_OOPS)
> -		return NOTIFY_DONE;
> 
>  	/*
> -	 * Hyper-V should be notified only once about a panic.  If we will be
> -	 * doing hv_kmsg_dump() with kmsg data later, don't do the notification
> -	 * here.
> +	 * Hyper-V should be notified only once about a panic/die. If we will
> +	 * be calling hv_kmsg_dump() later with kmsg data, don't do the
> +	 * notification here.
>  	 */
>  	if (hyperv_report_reg())
> -		hyperv_report_panic(regs, val, true);
> +		hyperv_report_panic(regs, val, is_die);
> +
>  	return NOTIFY_DONE;
>  }
> 
> -static struct notifier_block hyperv_die_block = {
> -	.notifier_call = hyperv_die_event,
> -};
> -static struct notifier_block hyperv_panic_block = {
> -	.notifier_call = hyperv_panic_event,
> -};
> -
>  static const char *fb_mmio_name = "fb_range";
>  static struct resource *fb_mmio;
>  static struct resource *hyperv_mmio;
> @@ -1538,16 +1558,17 @@ static int vmbus_bus_init(void)
>  		if (hyperv_crash_ctl & HV_CRASH_CTL_CRASH_NOTIFY_MSG)
>  			hv_kmsg_dump_register();
> 
> -		register_die_notifier(&hyperv_die_block);
> +		register_die_notifier(&hyperv_die_report_block);
> +		atomic_notifier_chain_register(&panic_notifier_list,
> +						&hyperv_panic_report_block);
>  	}
> 
>  	/*
> -	 * Always register the panic notifier because we need to unload
> -	 * the VMbus channel connection to prevent any VMbus
> -	 * activity after the VM panics.
> +	 * Always register the vmbus unload panic notifier because we
> +	 * need to shut the VMbus channel connection on panic.
>  	 */
>  	atomic_notifier_chain_register(&panic_notifier_list,
> -			       &hyperv_panic_block);
> +			       &hyperv_panic_vmbus_unload_block);
> 
>  	vmbus_request_offers();
> 
> @@ -2776,15 +2797,17 @@ static void __exit vmbus_exit(void)
> 
>  	if (ms_hyperv.misc_features &
> HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
>  		kmsg_dump_unregister(&hv_kmsg_dumper);
> -		unregister_die_notifier(&hyperv_die_block);
> +		unregister_die_notifier(&hyperv_die_report_block);
> +		atomic_notifier_chain_unregister(&panic_notifier_list,
> +						&hyperv_panic_report_block);
>  	}
> 
>  	/*
> -	 * The panic notifier is always registered, hence we should
> +	 * The vmbus panic notifier is always registered, hence we should
>  	 * also unconditionally unregister it here as well.
>  	 */
>  	atomic_notifier_chain_unregister(&panic_notifier_list,
> -					 &hyperv_panic_block);
> +					&hyperv_panic_vmbus_unload_block);
> 
>  	free_page((unsigned long)hv_panic_page);
>  	unregister_sysctl_table(hv_ctl_table_hdr);
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> index e1b65a01fb96..9234d31d4305 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -1216,7 +1216,15 @@ static int hvfb_probe(struct hv_device *hdev,
>  	par->fb_ready = true;
> 
>  	par->synchronous_fb = false;
> +
> +	/*
> +	 * We need to be sure this panic notifier runs _before_ the
> +	 * vmbus disconnect, so order it by priority. It must execute
> +	 * before the function hv_panic_vmbus_unload() [drivers/hv/vmbus_drv.c],
> +	 * which is almost at the end of list, with priority = INT_MIN + 1.
> +	 */
>  	par->hvfb_panic_nb.notifier_call = hvfb_on_panic;
> +	par->hvfb_panic_nb.priority = INT_MIN + 10,
>  	atomic_notifier_chain_register(&panic_notifier_list,
>  				       &par->hvfb_panic_nb);
> 
> --
> 2.37.2

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