iommu/amd: Add amd_iommu=force_enable option

Add this option to enable the IOMMU on platforms like AMD Stoney,
where the kernel usually disables it because it may cause problems in
some scenarios.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20210603130203.29016-1-joro@8bytes.org

Documentation/admin-guide/kernel-parameters.txt

@@ -301,6 +301,9 @@
 					  allowed anymore to lift isolation
 					  requirements as needed. This option
 					  does not override iommu=pt
+			force_enable - Force enable the IOMMU on platforms known
+				       to be buggy with IOMMU enabled. Use this
+				       option with care.
 
 	amd_iommu_dump=	[HW,X86-64]
 			Enable AMD IOMMU driver option to dump the ACPI table

drivers/iommu/amd/init.c

@@ -153,7 +153,8 @@ int amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_VAPIC;
 static int amd_iommu_xt_mode = IRQ_REMAP_XAPIC_MODE;
 
 static bool amd_iommu_detected;
-static bool __initdata amd_iommu_disabled;
+static bool amd_iommu_disabled __initdata;
+static bool amd_iommu_force_enable __initdata;
 static int amd_iommu_target_ivhd_type;
 
 u16 amd_iommu_last_bdf;			/* largest PCI device id we have
@@ -2834,6 +2835,9 @@ static bool detect_ivrs(void)
 
 	acpi_put_table(ivrs_base);
 
+	if (amd_iommu_force_enable)
+		goto out;
+
 	/* Don't use IOMMU if there is Stoney Ridge graphics */
 	for (i = 0; i < 32; i++) {
 		u32 pci_id;
@@ -2845,6 +2849,7 @@ static bool detect_ivrs(void)
 		}
 	}
 
+out:
 	/* Make sure ACS will be enabled during PCI probe */
 	pci_request_acs();
 
@@ -3100,6 +3105,8 @@ static int __init parse_amd_iommu_options(char *str)
 	for (; *str; ++str) {
 		if (strncmp(str, "fullflush", 9) == 0)
 			amd_iommu_unmap_flush = true;
+		if (strncmp(str, "force_enable", 12) == 0)
+			amd_iommu_force_enable = true;
 		if (strncmp(str, "off", 3) == 0)
 			amd_iommu_disabled = true;
 		if (strncmp(str, "force_isolation", 15) == 0)