linux-kernel - Re: [RFC PATCH 03/41] perf: Set exclude_guest onto nmi

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Message-ID: <Zhgycg0Bu0kL9D_W@google.com>
Date: Thu, 11 Apr 2024 11:56:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiong Zhang <xiong.y.zhang@...ux.intel.com>
Cc: pbonzini@...hat.com, peterz@...radead.org, mizhang@...gle.com, 
	kan.liang@...el.com, zhenyuw@...ux.intel.com, dapeng1.mi@...ux.intel.com, 
	jmattson@...gle.com, kvm@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhiyuan.lv@...el.com, eranian@...gle.com, 
	irogers@...gle.com, samantha.alt@...el.com, like.xu.linux@...il.com, 
	chao.gao@...el.com, Xiong Zhang <xiong.y.zhang@...el.com>
Subject: Re: [RFC PATCH 03/41] perf: Set exclude_guest onto nmi_watchdog

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> From: Xiong Zhang <xiong.y.zhang@...el.com>
> 
> 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@...el.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  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
>