Re: [RFC PATCH 03/41] perf: Set exclude_guest onto nmi_watchdog

[Sean Christopherson]

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> From: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> 
> The perf event for NMI watchdog is per cpu pinned system wide event,
> if such event doesn't have exclude_guest flag, it will be put into
> error state once guest with passthrough PMU starts, this breaks
> NMI watchdog function totally.
> 
> This commit adds exclude_guest flag for this perf event, so this perf
> event is stopped during VM running, but it will continue working after
> VM exit. In this way the NMI watchdog can not detect hardlockups during
> VM running, it still breaks NMI watchdog function a bit. But host perf
> event must be stopped during VM with passthrough PMU running, current
> no other reliable method can be used to replace perf event for NMI
> watchdog.

As mentioned in the cover letter, I think this is backwards, and mediated PMU
support should be disallowed if kernel-priority things like the watchdog are in
use.

Doubly so because this patch affects _everything_, not just systems with VMs
that have a mediated PMU.

> Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  kernel/watchdog_perf.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
> index 8ea00c4a24b2..c8ba656ff674 100644
> --- a/kernel/watchdog_perf.c
> +++ b/kernel/watchdog_perf.c
> @@ -88,6 +88,7 @@ static struct perf_event_attr wd_hw_attr = {
>  	.size		= sizeof(struct perf_event_attr),
>  	.pinned		= 1,
>  	.disabled	= 1,
> +	.exclude_guest  = 1,
>  };
>  
>  /* Callback function for perf event subsystem */
> -- 
> 2.34.1
>