Xen project Mailing List

Hi,

I have isolated what appears to be an incompatibility between Xen's Intel vPMU implementation and Linux's Alder Lake hybrid PMU initialization.

Host:

Qubes OS

Xen 4.17.5

vpmu=1

/sys/hypervisor/pmu/pmu_mode = self

/sys/hypervisor/pmu/pmu_features = 0x0

Guest:

PVH

Fedora 43

Linux 7.0.11 (also reproduced with 6.12)

Alder Lake hybrid PMU detected

With an unmodified kernel, Linux reports:

Performance Events: Alderlake Hybrid events, Intel PMU driver.

but then faults while enabling the P-core PMU:

unchecked MSR access error: WRMSR to 0x38f

tried to write 0x000100070000003f

__intel_pmu_enable_all

0x38f is IA32_PERF_GLOBAL_CTRL. The failing value contains bit 48 (GLOBAL_CTRL_EN_PERF_METRICS).

On Alder Lake, Linux explicitly enables this for the hybrid_big PMU in arch/x86/events/intel/core.c:

} else if (pmu->pmu_type & hybrid_big) {

pmu->intel_cap.perf_metrics = 1;

Xen 4.17's Intel vPMU appears not to include bit 48 in the accepted global control mask and consequently rejects the MSR write.

The practical result is that ordinary hardware counters are also unusable. For example, perf stat reported zero cycles/instructions.

As a diagnostic workaround I built Linux 7.0.11 with only:

pmu->intel_cap.perf_metrics = 1;

changed to:

pmu->intel_cap.perf_metrics = 0;

After booting that kernel, the WRMSR fault disappears completely and ordinary PMU counters immediately work:

12,228,346,996 cpu_core/cycles/

27,471,544,939 cpu_core/instructions/

A repeated run produced approximately the same instruction count and sensible cycle counts.

So the issue appears causal: Linux enables Alder Lake P-core PERF_METRICS through IA32_PERF_GLOBAL_CTRL[48], Xen rejects that bit, and PMU initialization is effectively broken as a consequence.

I have not yet tested current xen.git, so this may already have been addressed after 4.17. Is bit 48 / Intel PERF_METRICS supported by the current vPMU implementation? If not, I think Xen should either virtualize it or expose PMU capabilities such that the guest does not attempt to enable it.

I can provide further traces or test a Xen patch if useful.

[BUG] Intel Alder Lake guest PMU unusable because Xen vPMU rejects IA32_PERF_GLOBAL_CTRL bit 48