Putting HYPERV_FEAT_SYNDBG entry under "#ifdef CONFIG_SYNDBG" in
'kvm_hyperv_properties' array is wrong: as HYPERV_FEAT_SYNDBG is not
the highest feature number, the result is an empty (zeroed) entry in
the array (and not a skipped entry!). hyperv_feature_supported() is
designed to check that all CPUID bits are set but for a zeroed
feature in 'kvm_hyperv_properties' it returns 'true' so QEMU considers
HYPERV_FEAT_SYNDBG as always supported, regardless of whether KVM host
actually supports it.

To fix the issue, leave HYPERV_FEAT_SYNDBG's definition in
'kvm_hyperv_properties' array, there's nothing wrong in having it defined
even when 'CONFIG_SYNDBG' is not set. Instead, put "hv-syndbg" CPU property
under '#ifdef CONFIG_SYNDBG' to alter the existing behavior when the flag
is silently skipped in !CONFIG_SYNDBG builds.

Leave an 'assert' sentinel in hyperv_feature_supported() making sure there
are no 'holes' or improperly defined features in 'kvm_hyperv_properties'.

Fixes: d8701185f40c ("hw: hyperv: Initial commit for Synthetic Debugging device")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 target/i386/cpu.c     |  2 ++
 target/i386/kvm/kvm.c | 11 +++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 358d9c0a655a..f5fac3744173 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7842,8 +7842,10 @@ static Property x86_cpu_properties[] = {
                       HYPERV_FEAT_TLBFLUSH_DIRECT, 0),
     DEFINE_PROP_ON_OFF_AUTO("hv-no-nonarch-coresharing", X86CPU,
                             hyperv_no_nonarch_cs, ON_OFF_AUTO_OFF),
+#ifdef CONFIG_SYNDBG
     DEFINE_PROP_BIT64("hv-syndbg", X86CPU, hyperv_features,
                       HYPERV_FEAT_SYNDBG, 0),
+#endif
     DEFINE_PROP_BOOL("hv-passthrough", X86CPU, hyperv_passthrough, false),
     DEFINE_PROP_BOOL("hv-enforce-cpuid", X86CPU, hyperv_enforce_cpuid, false),
 
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 11b8177eff21..2fcb1f6673d8 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -945,7 +945,6 @@ static struct {
              .bits = HV_DEPRECATING_AEOI_RECOMMENDED}
         }
     },
-#ifdef CONFIG_SYNDBG
     [HYPERV_FEAT_SYNDBG] = {
         .desc = "Enable synthetic kernel debugger channel (hv-syndbg)",
         .flags = {
@@ -954,7 +953,6 @@ static struct {
         },
         .dependencies = BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_RELAXED)
     },
-#endif
     [HYPERV_FEAT_MSR_BITMAP] = {
         .desc = "enlightened MSR-Bitmap (hv-emsr-bitmap)",
         .flags = {
@@ -1206,6 +1204,13 @@ static bool hyperv_feature_supported(CPUState *cs, int feature)
     uint32_t func, bits;
     int i, reg;
 
+    /*
+     * kvm_hyperv_properties needs to define at least one CPUID flag which
+     * must be used to detect the feature, it's hard to say whether it is
+     * supported or not otherwise.
+     */
+    assert(kvm_hyperv_properties[feature].flags[0].func);
+
     for (i = 0; i < ARRAY_SIZE(kvm_hyperv_properties[feature].flags); i++) {
 
         func = kvm_hyperv_properties[feature].flags[i].func;
@@ -3391,13 +3396,11 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
                 kvm_msr_entry_add(cpu, HV_X64_MSR_TSC_EMULATION_STATUS,
                                   env->msr_hv_tsc_emulation_status);
             }
-#ifdef CONFIG_SYNDBG
             if (hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNDBG) &&
                 has_msr_hv_syndbg_options) {
                 kvm_msr_entry_add(cpu, HV_X64_MSR_SYNDBG_OPTIONS,
                                   hyperv_syndbg_query_options());
             }
-#endif
         }
         if (hyperv_feat_enabled(cpu, HYPERV_FEAT_VAPIC)) {
             kvm_msr_entry_add(cpu, HV_X64_MSR_APIC_ASSIST_PAGE,
-- 
2.41.0