pdx: introduce command line compression toggle

Introduce a command line option to allow disabling PDX compression.  The
disabling is done by turning pfn_pdx_add_region() into a no-op, so when
attempting to initialize the selected compression algorithm the array of
ranges to compress is empty.

Signed-off-by: Roger Pau Monné <roger.pau@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

docs/misc/xen-command-line.pandoc

@@ -2072,6 +2072,15 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for
 those not subject to XPTI (`no-xpti`). The feature is used only in case
 INVPCID is supported and not disabled via `invpcid=false`.
 
+### pdx-compress
+> `= <boolean>`
+
+> Default: `true` if CONFIG_PDX_NONE is unset
+
+Only relevant when the hypervisor is build with PFN PDX compression. Controls
+whether Xen will engage in PFN compression.  The algorithm used for PFN
+compression is selected at build time from Kconfig.
+
 ### ple_gap
 > `= <integer>`
 

xen/common/pdx.c

@@ -19,6 +19,7 @@
 #include <xen/mm.h>
 #include <xen/bitops.h>
 #include <xen/nospec.h>
+#include <xen/param.h>
 #include <xen/pfn.h>
 #include <xen/sections.h>
 
@@ -76,9 +77,13 @@ static struct pfn_range {
 } ranges[MAX_PFN_RANGES] __initdata;
 static unsigned int __initdata nr_ranges;
 
+static bool __initdata pdx_compress = true;
+boolean_param("pdx-compress", pdx_compress);
+
 void __init pfn_pdx_add_region(paddr_t base, paddr_t size)
 {
-    if ( !size )
+    /* Without ranges there's no PFN compression. */
+    if ( !size || !pdx_compress )
         return;
 
     if ( nr_ranges >= ARRAY_SIZE(ranges) )
@@ -215,6 +220,13 @@ void __init pfn_pdx_compression_setup(paddr_t base)
     unsigned int i, j, bottom_shift = 0, hole_shift = 0;
     unsigned long mask = pdx_init_mask(base) >> PAGE_SHIFT;
 
+    if ( !nr_ranges )
+    {
+        printk(XENLOG_DEBUG "PFN compression disabled%s\n",
+               pdx_compress ? ": no ranges provided" : "");
+        return;
+    }
+
     if ( nr_ranges > ARRAY_SIZE(ranges) )
     {
         printk(XENLOG_WARNING