[Xen-devel] [PATCH v2 4/4] xen/dom0: Add a dom0-iommu=none option

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

To fix a corner case with command line parsing, cmdline_strcmp() is
introduced.  Because we no longer tokenise comma separated list with NUL's,
strcmp(line, "opt") doesn't work for a string in the middle of the comma
separated list, and strncmp("opt", s, ss - s) matches "o", "op" and "opt" on
the command line.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

Slightly RFC.  I've been carrying this patch locally for ages, but decided
that the approach is more likely to be accepted:

    diff --git a/xen/drivers/passthrough/x86/iommu.c 
b/xen/drivers/passthrough/x86/iommu.c
    index c68a722..87f0fd9 100644
    --- a/xen/drivers/passthrough/x86/iommu.c
    +++ b/xen/drivers/passthrough/x86/iommu.c
    @@ -117,8 +117,6 @@ int arch_iommu_populate_page_table(struct domain *d)

     void __hwdom_init arch_iommu_check_autotranslated_hwdom(struct domain *d)
     {
    -    if ( !iommu_enabled )
    -        panic("Presently, iommu must be enabled for PVH hardware 
domain\n");
     }

     int arch_iommu_domain_init(struct domain *d)

v2:
 * Retain `none` as opposed to repurposing `passthrough`.
 * Update cmdline_strcmp() to look only for commas.
---
 docs/misc/xen-command-line.markdown |  8 +++++++-
 xen/common/kernel.c                 | 20 ++++++++++++++++++++
 xen/drivers/passthrough/iommu.c     |  5 ++++-
 xen/include/xen/lib.h               |  7 +++++++
 4 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 3a9af17..915d25c 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -664,7 +664,7 @@ Controls for how dom0 is constructed on x86 systems.
     hardware is not HAP-capable.
 
 ### dom0-iommu
-> `= List of [ passthrough=<bool>, strict=<bool>, map-reserved=<bool> ]`
+> `= List of [ passthrough=<bool>, strict=<bool>, map-reserved=<bool>, none ]`
 
 Controls for the dom0 IOMMU setup.
 
@@ -706,6 +706,12 @@ Controls for the dom0 IOMMU setup.
     This option is enabled by default on x86 systems, and invalid on ARM
     systems.
 
+*   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>`
 
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 5766a0f..af96850 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -271,6 +271,26 @@ int parse_boolean(const char *name, const char *s, const 
char *e)
     return -1;
 }
 
+int cmdline_strcmp(const char *frag, const char *name)
+{
+    while ( 1 )
+    {
+        int res = (*frag - *name);
+
+        if ( res || *name == '\0' )
+        {
+            /* NUL in 'name' matching a comma in 'frag' implies success. */
+            if ( *name == '\0' && *frag == ',' )
+                res = 0;
+
+            return res;
+        }
+
+        frag++;
+        name++;
+    }
+}
+
 unsigned int tainted;
 
 /**
diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
index d2ee2ee..94b5f4c 100644
--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -59,6 +59,7 @@ 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;
 int8_t __hwdom_initdata iommu_hwdom_reserved = -1;
@@ -155,6 +156,8 @@ static int __init parse_dom0_iommu_param(const char *s)
             iommu_hwdom_strict = 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;
 
@@ -183,7 +186,7 @@ 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);
diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 972fc84..1540fe0 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -79,6 +79,13 @@ int parse_bool(const char *s, const char *e);
  */
 int parse_boolean(const char *name, const char *s, const char *e);
 
+/**
+ * Very similar to strcmp(), but will declare a match if the NUL in 'name'
+ * lines up with comma in 'frag'.  Designed for picking exact string matches
+ * out of a comma-separated command line fragment.
+ */
+int cmdline_strcmp(const char *frag, const char *name);
+
 /*#define DEBUG_TRACE_DUMP*/
 #ifdef DEBUG_TRACE_DUMP
 extern void debugtrace_dump(void);
-- 
2.1.4


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