amd-iommu: set event buffer head and tail to 0 manually

These registers may contain values from previous kernels. So reset them
to known values before enable the event buffer again.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

arch/x86/kernel/amd_iommu_init.c

@@ -486,6 +486,10 @@ static void iommu_enable_event_buffer(struct amd_iommu *iommu)
 	memcpy_toio(iommu->mmio_base + MMIO_EVT_BUF_OFFSET,
 		    &entry, sizeof(entry));
 
+	/* set head and tail to zero manually */
+	writel(0x00, iommu->mmio_base + MMIO_EVT_HEAD_OFFSET);
+	writel(0x00, iommu->mmio_base + MMIO_EVT_TAIL_OFFSET);
+
 	iommu_feature_enable(iommu, CONTROL_EVT_LOG_EN);
 }