linux-kernel - RE: [PATCH V2 3/6] x86/Hyper-V: Trigger crash enlightenment only once during system crash.

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Date:   Mon, 23 Mar 2020 17:15:31 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "ltykernel@...il.com" <ltykernel@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <liuwe@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>
Subject: RE: [PATCH V2 3/6] x86/Hyper-V: Trigger crash enlightenment only once
 during  system crash.

From: Tianyu Lan <Tianyu.Lan@...rosoft.com> Sent: Monday, March 23, 2020 6:09 AM
> 
> When a guest VM panics, Hyper-V should be notified only once via the
> crash synthetic MSRs.  Current Linux code might write these crash MSRs
> twice during a system panic:
> 1) hyperv_panic/die_event() calling hyperv_report_panic()
> 2) hv_kmsg_dump() calling hyperv_report_panic_msg()
> 
> Fix this by not calling hyperv_report_panic() if a kmsg dump has been
> successfully registered.  The notification will happen later via
> hyperv_report_panic_msg().
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> ---
> Change since v1:
> 	- Update commit log
> ---
>  drivers/hv/vmbus_drv.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 3a0472c8b7ae..d73fa8aa00a3 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -55,7 +55,12 @@ static int hyperv_panic_event(struct notifier_block *nb, unsigned
> long val,
> 
>  	vmbus_initiate_unload(true);
> 
> -	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
> +	/*
> +	 * Crash notify only can be triggered once. If crash notify
> +	 * message is available, just report kmsg to crash buffer.
> +	 */

Just to clarify the above comment, let me suggest:

	/*
	 * Hyper-V should be notified only once about a panic.  If we will be
	 * doing hyperv_report_panic_msg() later with kmsg data, don't do
	 * the notification here.
	 */

> +	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE
> +	    && !hv_panic_page) {
>  		regs = current_pt_regs();
>  		hyperv_report_panic(regs, val);
>  	}
> @@ -68,7 +73,12 @@ static int hyperv_die_event(struct notifier_block *nb, unsigned long
> val,
>  	struct die_args *die = (struct die_args *)args;
>  	struct pt_regs *regs = die->regs;
> 
> -	hyperv_report_panic(regs, val);
> +	/*
> +	 * Crash notify only can be triggered once. If crash notify
> +	 * message is available, just report kmsg to crash buffer.
> +	 */

Same suggested clarification to the comment applies here.

> +	if (!hv_panic_page)
> +		hyperv_report_panic(regs, val);
>  	return NOTIFY_DONE;
>  }
> 
> --
> 2.14.5