tonic ce9416a9c2 arch: x86_64: advertise mandatory Hyper-V partition privileges ...

The Microsoft "Requirements for Implementing the Microsoft Hypervisor
Interface" document marks exactly two privileges in CPUID leaf
0x40000003 EAX as "Must be set": AccessHypercallMsrs (bit 5) and
AccessVpIndex (bit 6). Cloud Hypervisor advertised neither.

Without bit 5, Windows guests abort enlightened-mode initialization
before timer-API selection: HalpHvTimerApi is left NULL and every
QueryPerformanceCounter call falls back to reading
HV_X64_MSR_TIME_REF_COUNT (0x40000020), costing one VM exit per call.
The guest never writes HV_X64_MSR_REFERENCE_TSC (0x40000021) to enable
the reference TSC page, even though AccessPartitionReferenceTsc (bit 9)
is advertised.

With both bits set, Windows 10 22H2 and Windows 11 25H2 guests enable
the reference TSC page at boot. Measured QueryPerformanceCounter
throughput on a nested-KVM host went from ~71K calls/sec (14 us/call,
one MSR exit each) to ~1.3M calls/sec (free, no exits); on bare metal
from ~390K to ~1.9M calls/sec. Guest idle CPU and interrupt-service
time drop correspondingly.

Both MSR ranges are already handled in-kernel by KVM unconditionally,
so no backend change is needed. Bisection across the full delta to
QEMU's Hyper-V CPUID layout (vendor ID, max leaf, leaves 4-6 contents,
build number) shows bit 5 is the only load-bearing change; bit 6 is
included per the conformance document's mandate.

Signed-off-by: Tonic Li <tonic@simular.ai>
Signed-off-by: tonic <tonicbupt@gmail.com>

2026-06-15 13:54:01 +00:00

..

src

arch: x86_64: advertise mandatory Hyper-V partition privileges

2026-06-15 13:54:01 +00:00

Cargo.toml

arch: Helper functions for u32 hex (de-) serialization

2026-05-28 15:19:15 +00:00