linux-kernel - Re: [PATCH v4] perf/x86/amd: Don't touch the Host-only bit inside the guest hypervisor

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Date:   Thu, 24 Mar 2022 11:42:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dongli Si <kvmx86@...il.com>
Cc:     joro@...tes.org, liam.merwick@...cle.com, kim.phillips@....com,
        mingo@...nel.org, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] perf/x86/amd: Don't touch the Host-only bit inside
 the guest hypervisor

On Sun, Mar 20, 2022 at 08:21:06AM +0800, Dongli Si wrote:

> @@ -1027,7 +1028,8 @@ void amd_pmu_enable_virt(void)
>  {
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>  
> -	cpuc->perf_ctr_virt_mask = 0;
> +	if (hypervisor_is_type(X86_HYPER_NATIVE))
> +		cpuc->perf_ctr_virt_mask = 0;

So I had to go and read the original commit to figure out wth this code
is supposed to be doing. I'm thinking this all can use a wee comment.

Also, the above is very ambiguous as to what it does. Specifically if we
don't set perf_ctr_virt_mask, then what is the actual value (yes, I
know, but its not very clear is it). Also, if we don't in fact change
perf_ctr_virt_mask then these following two lines:

>  
>  	/* Reload all events */
>  	amd_pmu_disable_all();
	amd_pmu_enable_all(0);

are somewhat completely pointless, no?

So can we please get a patch that clarifies things instead of making
things even more obscure?