Re: [PATCH 2/2] hw/i386/amd_iommu: Fix xtsup when vcpus < 255

[Alejandro Jimenez]

On 4/10/25 2:44 AM, Sairaj Kodilkar wrote:

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

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

Booting guest in x2apic mode, amd-iommu,xtsup=on and <= 255 vCPUs is
broken as it fails to call kvm_enable_x2apic().

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.jime...@oracle.com>
Cc: Philippe Mathieu-Daudé <phi...@linaro.org>
Cc: Joao Martins <joao.m.mart...@oracle.com>
Signed-off-by: Vasant Hegde <vasant.he...@amd.com>

Tested-by: Alejandro Jimenez <alejandro.j.jime...@oracle.com>

---
  hw/i386/amd_iommu.c | 8 ++++++++
  1 file changed, 8 insertions(+)

diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
index df8ba5d39ada..af85706b8a0d 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);
  }