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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/09/2026 1:56 pm, Mr. Hoorn wrote:
> 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.

Thankyou for the report, but this won't have been fixed.

Xen has no real knowledge of Hybrid; this is a known and out standing issue.

~Andrew