Commit 1e682edf3f for qemu.org

commit 1e682edf3fc3c5dcad9ba0cee214dbf54a3cb462
Author: Akshey Taneja <aksheynew.tanejanew@gmail.com>
Date:   Thu Oct 15 10:02:16 2026 -0700

    whpx: i386: fall back to xAPIC when the MSR exit bitmap is unavailable

    WHvPartitionPropertyCodeX64MsrExitBitmap is not implemented on Windows
    10 version 1809 and earlier, 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 version 1809 (build 17763)

    Signed-off-by: Akshey Taneja <aksheynew.tanejanew@gmail.com>

    [Patch edited, version 1903 is the first release of Windows 10 with this API]
    Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>

diff --git a/target/i386/whpx/whpx-all.c b/target/i386/whpx/whpx-all.c
index b386368f1d..df1c772eb3 100644
--- a/target/i386/whpx/whpx-all.c
+++ b/target/i386/whpx/whpx-all.c
@@ -57,6 +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[] = {

@@ -2981,6 +2982,16 @@ 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)
 {
@@ -2988,7 +2999,8 @@ 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);
@@ -3396,10 +3408,20 @@ int whpx_accel_init(AccelState *as, MachineState *ms)
             WHvPartitionPropertyCodeX64MsrExitBitmap,
             &prop,
             sizeof(WHV_PARTITION_PROPERTY));
+    /*
+     * Earlier Windows 10 releases not implement this property.
+     * It is implemented starting from Windows 10 version 1903 (18362).
+     */
     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(