WHvPartitionPropertyCodeX64MsrExitBitmap is not implemented on Windows
10, where WHvSetPartitionProperty() returns WHV_E_UNKNOWN_PROPERTY
(0x80370302) and whpx_accel_init() aborts with -EINVAL. QEMU started
requesting it in ba1bda25b7 ("whpx: i386: x2apic emulation").Treating the failure as non-fatal is not sufficient on its own: whpx_default_props unconditionally enables x2apic, and x2APIC is reached through MSRs, so without MSR exits the userspace LAPIC never observes the guest's APIC accesses and the guest hangs immediately after the firmware hands over. Observed as a black screen with an Ubuntu 24.04 guest that otherwise boots in about 40 seconds. Record whether the MSR exit bitmap was accepted and, when it was not, apply x2apic=off so the guest uses the MMIO based xAPIC, which is still intercepted normally. Reported-on: Windows 10 Enterprise LTSC 17763, Intel i7-6700 Signed-off-by: Akshey Taneja <[email protected]> --- target/i386/whpx/whpx-all.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-) diff --git a/target/i386/whpx/whpx-all.c b/target/i386/whpx/whpx-all.c index 9a9fc7d974..3e59aacf31 100644 --- a/target/i386/whpx/whpx-all.c +++ b/target/i386/whpx/whpx-all.c @@ -57,7 +57,7 @@ #define HV_X64_MSR_GUEST_IDLE 0x400000f0 static bool is_modern_os = true; - +static bool whpx_msr_exits_available = true; static const WHV_REGISTER_NAME whpx_register_names[] = { /* X64 General purpose registers */ @@ -2889,6 +2889,16 @@ static PropValue whpx_default_props[] = { { NULL, NULL }, }; +/* + * x2APIC is reached through MSRs, so emulating it in userspace requires the + * MSR exit bitmap. Hosts without that property must fall back to xAPIC, + * which is MMIO based and therefore still intercepted. + */ +static PropValue whpx_no_msr_exit_props[] = { + { "x2apic", "off" }, + { NULL, NULL }, +}; + void whpx_cpu_instance_init(CPUState *cs) { @@ -2896,7 +2906,8 @@ void whpx_cpu_instance_init(CPUState *cs) X86CPUClass *xcc = X86_CPU_GET_CLASS(cpu); host_cpu_instance_init(cpu); - x86_cpu_apply_props(cpu, whpx_default_props); + x86_cpu_apply_props(cpu, whpx_msr_exits_available ? whpx_default_props + : whpx_no_msr_exit_props); if (xcc->max_features) { whpx_cpu_max_instance_init(cpu); @@ -3344,10 +3355,22 @@ int whpx_accel_init(AccelState *as, MachineState *ms) WHvPartitionPropertyCodeX64MsrExitBitmap, &prop, sizeof(WHV_PARTITION_PROPERTY)); + /* + * Windows 10 does not implement this property. Without the MSR exits the + * guest simply gets the behaviour QEMU had before commit ba1bda25b7 + * ("whpx: i386: x2apic emulation") started asking for them, so treat an + * unknown property as non-fatal rather than refusing to start. + */ if (FAILED(hr)) { - error_report("WHPX: Failed to set MSR exit bitmap, hr=%08lx", hr); - ret = -EINVAL; - goto error; + if (hr == WHV_E_UNKNOWN_PROPERTY) { + whpx_msr_exits_available = false; + warn_report("WHPX: MSR exit bitmap unsupported on this Windows " + "version, falling back to xAPIC"); + } else { + error_report("WHPX: Failed to set MSR exit bitmap, hr=%08lx", hr); + ret = -EINVAL; + goto error; + } } hr = whp_dispatch.WHvSetPartitionProperty( -- 2.50.1.windows.1
