linux-kernel - [PATCH] arch/x86/events/amd/core.c: Return -ENODEV when CPU does not have PERFCTL

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Message-Id: <20221027133511.161922-1-lyan@digtalocean.com>
Date:   Thu, 27 Oct 2022 09:35:11 -0400
From:   Liang Yan <lyan@...italocean.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] arch/x86/events/amd/core.c: Return -ENODEV when CPU does not have PERFCTL_CORE bit

After disabling cpu.perfctr_core in qemu, I noticed that the guest kernel
still loads the pmu driver while the cpuid does not have perfctl_core.

The test is running on an EPYC Rome machine.
root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~# lscpu | grep perfctl
root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~#
root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~# dmesg | grep PMU
[    0.732097] Performance Events: AMD PMU driver.

By further looking,

==> init_hw_perf_events
    ==> amd_pmu_init
        ==> amd_core_pmu_init
            ==>
                if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
			return 0;

With returning 0, it will bypass amd_pmu_init and return 0 to
init_hw_perf_events, and continue the initialization.

I am not a perf expert and not sure if it is expected for AMD PMU,
otherwise, it would be nice to return -ENODEV instead.

New output after the change:
root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~# dmesg | grep PMU
[    0.531609] Performance Events: no PMU driver, software events only.

Signed-off-by: Liang Yan <lyan@...talocean.com>
---
 arch/x86/events/amd/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 8b70237c33f7..34d3d2944020 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -1335,7 +1335,7 @@ static int __init amd_core_pmu_init(void)
 	int i;

 	if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
-		return 0;
+		return -ENODEV;

 	/* Avoid calculating the value each time in the NMI handler */
 	perf_nmi_window = msecs_to_jiffies(100);
-- 
2.34.1