Windows with Hyper-V role enabled doesn't boot with 'hv-passthrough' when
no debugger is configured, this significantly limits the usefulness of the
feature as there's no support for subtracting Hyper-V features from CPU
flags at this moment (e.g. "-cpu host,hv-passthrough,-hv-syndbg" does not
work). While this is also theoretically fixable, 'hv-syndbg' is likely
very special and unneeded in the default set. Genuine Hyper-V doesn't seem
to enable it either.

Introduce 'skip_passthrough' flag to 'kvm_hyperv_properties' and use it as
one-off to skip 'hv-syndbg' when enabling features in 'hv-passthrough'
mode. Note, "-cpu host,hv-passthrough,hv-syndbg" can still be used if
needed.

As both 'hv-passthrough' and 'hv-syndbg' are debug features, the change
should not have any effect on production environments.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 docs/system/i386/hyperv.rst | 13 +++++++++----
 target/i386/kvm/kvm.c       |  7 +++++--
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/docs/system/i386/hyperv.rst b/docs/system/i386/hyperv.rst
index 2505dc4c86e0..009947e39141 100644
--- a/docs/system/i386/hyperv.rst
+++ b/docs/system/i386/hyperv.rst
@@ -262,14 +262,19 @@ Supplementary features
 ``hv-passthrough``
   In some cases (e.g. during development) it may make sense to use QEMU in
   'pass-through' mode and give Windows guests all enlightenments currently
-  supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU
-  flag.
+  supported by KVM.
 
   Note: ``hv-passthrough`` flag only enables enlightenments which are known to QEMU
   (have corresponding 'hv-' flag) and copies ``hv-spinlocks`` and ``hv-vendor-id``
   values from KVM to QEMU. ``hv-passthrough`` overrides all other 'hv-' settings on
-  the command line. Also, enabling this flag effectively prevents migration as the
-  list of enabled enlightenments may differ between target and destination hosts.
+  the command line.
+
+  Note: ``hv-passthrough`` does not enable ``hv-syndbg`` which can prevent certain
+  Windows guests from booting when used without proper configuration. If needed,
+  ``hv-syndbg`` can be enabled additionally.
+
+  Note: ``hv-passthrough`` effectively prevents migration as the list of enabled
+  enlightenments may differ between target and destination hosts.
 
 ``hv-enforce-cpuid``
   By default, KVM allows the guest to use all currently supported Hyper-V
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 2fcb1f6673d8..0c745562b667 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -823,6 +823,7 @@ static struct {
         uint32_t bits;
     } flags[2];
     uint64_t dependencies;
+    bool skip_passthrough;
 } kvm_hyperv_properties[] = {
     [HYPERV_FEAT_RELAXED] = {
         .desc = "relaxed timing (hv-relaxed)",
@@ -951,7 +952,8 @@ static struct {
             {.func = HV_CPUID_FEATURES, .reg = R_EDX,
              .bits = HV_FEATURE_DEBUG_MSRS_AVAILABLE}
         },
-        .dependencies = BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_RELAXED)
+        .dependencies = BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_RELAXED),
+        .skip_passthrough = true,
     },
     [HYPERV_FEAT_MSR_BITMAP] = {
         .desc = "enlightened MSR-Bitmap (hv-emsr-bitmap)",
@@ -1360,7 +1362,8 @@ bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
          * hv_build_cpuid_leaf() uses this info to build guest CPUIDs.
          */
         for (feat = 0; feat < ARRAY_SIZE(kvm_hyperv_properties); feat++) {
-            if (hyperv_feature_supported(cs, feat)) {
+            if (hyperv_feature_supported(cs, feat) &&
+                !kvm_hyperv_properties[feat].skip_passthrough) {
                 cpu->hyperv_features |= BIT(feat);
             }
         }
-- 
2.41.0