linux-kernel - Re: [PATCH] perf/x86/amd: cpu_hw_events::perf_ctr_virt

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Date:   Tue, 12 Apr 2022 09:25:30 +0800
From:   Dongli Si <kvmx86@...il.com>
To:     like.xu.linux@...il.com
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, jmattson@...gle.com,
        jolsa@...nel.org, joro@...tes.org, kim.phillips@....com,
        kvmx86@...il.com, liam.merwick@...cle.com,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        mark.rutland@....com, mingo@...nel.org, namhyung@...nel.org,
        peterz@...radead.org, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH] perf/x86/amd: cpu_hw_events::perf_ctr_virt_mask should only be used on host

On Mon 11 Apr 2022 22:29:18 +0800, Like Xu wrote:
> Or you can work it out to make nested vPMU functional on AMD.
Unless in the future kvm wants to emulate on L1 HV the behavior of not
count when HO bit is set and SVM is disabled, otherwise it doesn't make
sense to use perf_ctr_virt_mask in the guest to mask HO bit. At least for
now, this patch helps clarify what perf_ctr_virt_mask actually does.

> This is not a typical revert commit.
Thanks for pointing out the problem with my patch,
I will write another patch specifically to revert this commit.

> Please check the chronological order of the related commits and the motivations.
I know that commit df51fe7ea1c1c fixed the problem of use vPMU on old KVM,
but I think it's a speculative way and make things a little obscure,
because this #GP is actually a KVM problem rather than a guest problem,
I think it is the user's responsibility to update their host kernel.

> > +	/*
> > +	 * When SVM is disabled, set the Host-Only bit will cause the
> > +	 * performance counter to not work.
> It's ridiculous. Based on the AMD APM Table 13-3. Host/Guest Only Bits,
> the performance counter would count "Host events" rather than "not work".
You are wrong, you can test it on the host, and the description of the
commit 1018faa6cf23 also pointed out this problem, this is the result of an
experiment, AMD APM has not documented this problem.

I forgot to say this is the behavior on the host, I will improve this
comment to specify 'why' more clearly, like this:
	/*
	 * It turns out that when SVM is disabled on the host (L0), set the
	 * Host-Only bit will cause the performance counter to not count.
	 */

> Note, your proposal change should work on the L0, L1 and L2.
Yes, I tested it on L0, L1, L2 with 5.18-rc1 and it works as expected.

There is a related discussion here:
https://lore.kernel.org/all/20220320002106.1800166-1-sidongli1997@gmail.com/

Regards,
Dongli