xen/dom0: Add a dom0-iommu=none option

For development purposes, it is very convenient to boot Xen as a PVH guest,
with an XTF PV or PVH "dom0".  The edit-compile-go cycle is a matter of
seconds, and you can reasonably insert printk() debugging in places which
which would be completely infeasible when booting fully-fledged guests.

However, the PVH dom0 path insists on having a working IOMMU, which doesn't
exist when virtualised as a PVH guest, and isn't necessary for XTF anyway.

Introduce a developer mode to skip the IOMMU requirement.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

docs/misc/xen-command-line.pandoc

@@ -667,7 +667,7 @@ Controls for how dom0 is constructed on x86 systems.
 
 ### dom0-iommu
     = List of [ passthrough=<bool>, strict=<bool>, map-inclusive=<bool>,
-                map-reserved=<bool> ]
+                map-reserved=<bool>, none ]
 
 Controls for the dom0 IOMMU setup.
 
@@ -718,6 +718,12 @@ Controls for the dom0 IOMMU setup.
     subset of the correction by only mapping reserved memory regions rather
     than all non-RAM regions.
 
+*   The `none` option is intended for development purposes only, and skips
+    certain safety checks pertaining to the correct IOMMU configuration for
+    dom0 to boot.
+
+    Incorrect use of this option may result in a malfunctioning system.
+
 ### dom0_ioports_disable (x86)
 > `= List of <hex>-<hex>`
 

xen/drivers/passthrough/iommu.c

@@ -36,6 +36,7 @@ bool_t __read_mostly iommu_snoop = 1;
 bool_t __read_mostly iommu_qinval = 1;
 bool_t __read_mostly iommu_intremap = 1;
 
+static bool __hwdom_initdata iommu_hwdom_none;
 bool __hwdom_initdata iommu_hwdom_strict;
 bool __read_mostly iommu_hwdom_passthrough;
 bool __hwdom_initdata iommu_hwdom_inclusive;
@@ -131,6 +132,8 @@ static int __init parse_dom0_iommu_param(const char *s)
             iommu_hwdom_inclusive = val;
         else if ( (val = parse_boolean("map-reserved", s, ss)) >= 0 )
             iommu_hwdom_reserved = val;
+        else if ( !cmdline_strcmp(s, "none") )
+            iommu_hwdom_none = true;
         else
             rc = -EINVAL;
 
@@ -159,7 +162,7 @@ int iommu_domain_init(struct domain *d)
 
 static void __hwdom_init check_hwdom_reqs(struct domain *d)
 {
-    if ( !paging_mode_translate(d) )
+    if ( iommu_hwdom_none || !paging_mode_translate(d) )
         return;
 
     arch_iommu_check_autotranslated_hwdom(d);