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

[Liang Yan]

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@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~# lscpu | grep perfctl
root@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~#
root@ubuntu-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@ubuntu-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@xxxxxxxxxxxxxxx>
---
 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