x86/iommu: check for CMPXCHG16B when enabling IOMMU

All hardware with VT-d/AMD-Vi has CMPXCHG16B support. Check this at
initialisation time, and otherwise refuse to use the IOMMU.

If the local APICs support x2APIC mode the IOMMU support for interrupt
remapping will be checked earlier using a specific helper.  If no support
for CX16 is detected by that earlier hook disable the IOMMU at that point
and prevent further poking for CX16 later in the boot process, which would
also fail.

There's a possible corner case when running virtualized, and the underlying
hypervisor exposing an IOMMU but no CMPXCHG16B support.  In which case
ignoring the IOMMU is fine, albeit the most natural would be for the
underlying hypervisor to also expose CMPXCHG16B support if an IOMMU is
available to the VM.

Note this change only introduces the checks, but doesn't remove the now
stale checks for CX16 support sprinkled in the IOMMU code.  Further changes
will take care of that.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/drivers/passthrough/amd/iommu_intr.c

@@ -649,6 +649,19 @@ bool __init cf_check iov_supports_xt(void)
     if ( !iommu_enable || !iommu_intremap )
         return false;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        AMD_IOMMU_ERROR("no CMPXCHG16B support, disabling IOMMU\n");
+        /*
+         * Disable IOMMU support at once: there's no reason to check for CX16
+         * yet again when attempting to initialize IOMMU DMA remapping
+         * functionality or interrupt remapping without x2APIC support.
+         */
+        iommu_enable = false;
+        iommu_intremap = iommu_intremap_off;
+        return false;
+    }
+
     if ( amd_iommu_prepare(true) )
         return false;
 

xen/drivers/passthrough/amd/pci_amd_iommu.c

@@ -309,6 +309,12 @@ static int __init cf_check iov_detect(void)
     if ( !iommu_enable && !iommu_intremap )
         return 0;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        AMD_IOMMU_ERROR("no CMPXCHG16B support, disabling IOMMU\n");
+        return -ENODEV;
+    }
+
     if ( (init_done ? amd_iommu_init_late()
                     : amd_iommu_init(false)) != 0 )
     {

xen/drivers/passthrough/vtd/intremap.c

@@ -150,6 +150,19 @@ bool __init cf_check intel_iommu_supports_eim(void)
     if ( !iommu_qinval || !iommu_intremap || list_empty(&acpi_drhd_units) )
         return false;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_ERR VTDPREFIX "no CMPXCHG16B support, disabling IOMMU\n");
+        /*
+         * Disable IOMMU support at once: there's no reason to check for CX16
+         * yet again when attempting to initialize IOMMU DMA remapping
+         * functionality or interrupt remapping without x2APIC support.
+         */
+        iommu_enable = false;
+        iommu_intremap = iommu_intremap_off;
+        return false;
+    }
+
     /* We MUST have a DRHD unit for each IOAPIC. */
     for ( apic = 0; apic < nr_ioapics; apic++ )
         if ( !ioapic_to_drhd(IO_APIC_ID(apic)) )

xen/drivers/passthrough/vtd/iommu.c

@@ -2633,6 +2633,13 @@ static int __init cf_check vtd_setup(void)
     int ret;
     bool reg_inval_supported = true;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_ERR VTDPREFIX "no CMPXCHG16B support, disabling IOMMU\n");
+        ret = -ENODEV;
+        goto error;
+    }
+
     if ( list_empty(&acpi_drhd_units) )
     {
         ret = -ENODEV;