RE: [PATCH 3/4] x86/Hyper-V: Trigger crash enlightenment only once during system crash.

[Michael Kelley]

From: ltykernel@xxxxxxxxx <ltykernel@xxxxxxxxx>  Sent: Tuesday, March 17, 2020 6:25 AM
> 
> Hyper-V expects guest only triggers crash enlightenment.
> The second crash notify will be ignored by Hyper-V.
> 
> Current code may trigger crash enlightenment during system
> panic twice.
> 1) The enlightenment is triggered in hyperv_panic/die_event()
> via hyperv_report_panic().
> 2) hv_kmsg_dump() reports kmsg to host via hyperv_report_panic_msg().
> 
> Fix it. If kmsg dump is registered successfully, just report
> kmsg via hyperv_report_panic_msg() and not report register values
> via hyperv_report_panic().

Suggested wording improvements:

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@xxxxxxxxxxxxx>
> ---
>  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 b043efea092a..1787d6246251 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.
> +	 */
> +	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.
> +	 */
> +	if (!hv_panic_page)
> +		hyperv_report_panic(regs, val);
>  	return NOTIFY_DONE;
>  }
> 
> --
> 2.14.5