From: Vasant Hegde <vasant.hegde@amd.com>

If vCPUs > 255 then x86 common code (x86_cpus_init()) call kvm_enable_x2apic().
But if vCPUs <= 255 then the common code won't calls kvm_enable_x2apic().

This is because commit 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM
checks on XTSup feature") removed the call to kvm_enable_x2apic when xtsup
is "on", which break things when guest is booted with x2apic mode and
there are <= 255 vCPUs.

Fix this by adding back kvm_enable_x2apic() call when xtsup=on.

Fixes: 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature")
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
Message-Id: <20250516100535.4980-3-sarunkod@amd.com>
Fixes: 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature")
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
(cherry picked from commit 0f178860df3489a9d3c19a5f7f024e6aa6c26515)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
index df8ba5d39a..af85706b8a 100644
--- a/hw/i386/amd_iommu.c
+++ b/hw/i386/amd_iommu.c
@@ -1649,6 +1649,14 @@ static void amdvi_sysbus_realize(DeviceState *dev, Error **errp)
         exit(EXIT_FAILURE);
     }
 
+    if (s->xtsup) {
+        if (kvm_irqchip_is_split() && !kvm_enable_x2apic()) {
+            error_report("AMD IOMMU xtsup=on requires x2APIC support on "
+                          "the KVM side");
+            exit(EXIT_FAILURE);
+        }
+    }
+
     pci_setup_iommu(bus, &amdvi_iommu_ops, s);
     amdvi_init(s);
 }
-- 
2.47.2