iommu/amd: Disable iommu only if amd_iommu=off is specified

It's ok to disable iommu early in normal kernel or in kdump kernel when
amd_iommu=off is specified. While we should not disable it in kdump kernel
when on-flight dma is still on-going.
Signed-off-by: NBaoquan He <bhe@redhat.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>

drivers/iommu/amd_iommu_init.c

@@ -2499,7 +2499,8 @@ static int __init early_amd_iommu_init(void)
 		goto out;
 
 	/* Disable any previously enabled IOMMUs */
-	disable_iommus();
+	if (!is_kdump_kernel() || amd_iommu_disabled)
+		disable_iommus();
 
 	if (amd_iommu_irq_remap)
 		amd_iommu_irq_remap = check_ioapic_information();