diff -r 95d8788bf4be xen/drivers/passthrough/amd/iommu_init.c
--- a/xen/drivers/passthrough/amd/iommu_init.c	Mon Jan 12 10:23:55 2009 +0000
+++ b/xen/drivers/passthrough/amd/iommu_init.c	Tue Jan 13 13:45:08 2009 +0100
@@ -195,6 +195,10 @@ static void __init set_iommu_command_buf
                          IOMMU_CONTROL_COMMAND_BUFFER_ENABLE_MASK,
                          IOMMU_CONTROL_COMMAND_BUFFER_ENABLE_SHIFT, &entry);
     writel(entry, iommu->mmio_base+IOMMU_CONTROL_MMIO_OFFSET);
+
+    /*reset head and tail pointer */
+    writel(0x0, iommu->mmio_base + IOMMU_CMD_BUFFER_HEAD_OFFSET);
+    writel(0x0, iommu->mmio_base + IOMMU_CMD_BUFFER_TAIL_OFFSET);
 }
 
 static void __init register_iommu_exclusion_range(struct amd_iommu *iommu)
@@ -259,6 +263,10 @@ static void __init set_iommu_event_log_c
                          IOMMU_CONTROL_COMP_WAIT_INT_MASK,
                          IOMMU_CONTROL_COMP_WAIT_INT_SHIFT, &entry);
     writel(entry, iommu->mmio_base+IOMMU_CONTROL_MMIO_OFFSET);
+
+    /*reset head and tail pointer */
+    writel(0x0, iommu->mmio_base + IOMMU_EVENT_LOG_HEAD_OFFSET);
+    writel(0x0, iommu->mmio_base + IOMMU_EVENT_LOG_TAIL_OFFSET);
 }
 
 static int amd_iommu_read_event_log(struct amd_iommu *iommu, u32 event[])