x86/mm: allow for building without shadow mode support

Considering the complexity of the code, it seems to be a reasonable
thing to allow people to disable that code entirely even outside the
immediate need for this by the next patch.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
There is one XXX being added to the code - while it doesn't look like
we need to set v->arch.paging.mode in shadow_vcpu_init(), it may be
deemed more secure to set up a table with stub function pointers.

--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -32,9 +32,13 @@ x86 := y
 x86_32 := n
 x86_64 := y
 
+shadow-paging ?= y
+
 CFLAGS += -mno-red-zone -mno-sse -fpic
 CFLAGS += -fno-asynchronous-unwind-tables
 # -fvisibility=hidden reduces -fpic cost, if it's available
 ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
 CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
 endif
+
+CFLAGS-$(shadow-paging) += -DCONFIG_SHADOW_PAGING
--- a/xen/arch/x86/mm/paging.c
+++ b/xen/arch/x86/mm/paging.c
@@ -635,16 +635,16 @@ int paging_domain_init(struct domain *d,
      * don't want to leak any active log-dirty bitmaps */
     d->arch.paging.log_dirty.top = _mfn(INVALID_MFN);
 
-    /* The order of the *_init calls below is important, as the later
-     * ones may rewrite some common fields.  Shadow pagetables are the
-     * default... */
-    shadow_domain_init(d, domcr_flags);
-
-    /* ... but we will use hardware assistance if it's available. */
+    /*
+     * Shadow pagetables are the default, but we will use
+     * hardware assistance if it's available and enabled.
+     */
     if ( hap_enabled(d) )
         hap_domain_init(d);
+    else
+        rc = shadow_domain_init(d, domcr_flags);
 
-    return 0;
+    return rc;
 }
 
 /* vcpu paging struct initialization goes here */
@@ -822,12 +822,16 @@ int paging_enable(struct domain *d, u32 
  * and therefore its pagetables will soon be discarded */
 void pagetable_dying(struct domain *d, paddr_t gpa)
 {
+#ifdef CONFIG_SHADOW_PAGING
     struct vcpu *v;
 
     ASSERT(paging_mode_shadow(d));
 
     v = d->vcpu[0];
     v->arch.paging.mode->shadow.pagetable_dying(v, gpa);
+#else
+    BUG();
+#endif
 }
 
 /* Print paging-assistance info to the console */
--- a/xen/arch/x86/mm/shadow/Makefile
+++ b/xen/arch/x86/mm/shadow/Makefile
@@ -1,4 +1,5 @@
-obj-$(x86_64) += common.o guest_2.o guest_3.o guest_4.o
+obj-y                := none.o
+obj-$(shadow-paging) := common.o guest_2.o guest_3.o guest_4.o
 
 guest_%.o: multi.c Makefile
 	$(CC) $(CFLAGS) -DGUEST_PAGING_LEVELS=$* -c $< -o $@
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -43,7 +43,7 @@ DEFINE_PER_CPU(uint32_t,trace_shadow_pat
 
 /* Set up the shadow-specific parts of a domain struct at start of day.
  * Called for every domain from arch_domain_create() */
-void shadow_domain_init(struct domain *d, unsigned int domcr_flags)
+int shadow_domain_init(struct domain *d, unsigned int domcr_flags)
 {
     INIT_PAGE_LIST_HEAD(&d->arch.paging.shadow.freelist);
     INIT_PAGE_LIST_HEAD(&d->arch.paging.shadow.pinned_shadows);
@@ -57,6 +57,8 @@ void shadow_domain_init(struct domain *d
     d->arch.paging.shadow.oos_off = (domcr_flags & DOMCRF_oos_off) ?  1 : 0;
 #endif
     d->arch.paging.shadow.pagetable_dying_op = 0;
+
+    return 0;
 }
 
 /* Setup the shadow-specfic parts of a vcpu struct. Note: The most important
--- /dev/null
+++ b/xen/arch/x86/mm/shadow/none.c	2015-01-26 17:23:04.000000000 +0100
@@ -0,0 +1,70 @@
+#include <xen/mm.h>
+#include <asm/shadow.h>
+
+static int _enable_log_dirty(struct domain *d, bool_t log_global)
+{
+    BUG_ON(!is_pv_domain(d));
+    return -EOPNOTSUPP;
+}
+
+static int _disable_log_dirty(struct domain *d)
+{
+    BUG_ON(!is_pv_domain(d));
+    return -EOPNOTSUPP;
+}
+
+static void _clean_dirty_bitmap(struct domain *d)
+{
+    BUG_ON(!is_pv_domain(d));
+}
+
+int shadow_domain_init(struct domain *d, unsigned int domcr_flags)
+{
+    paging_log_dirty_init(d, _enable_log_dirty,
+                          _disable_log_dirty, _clean_dirty_bitmap);
+    return is_pv_domain(d) ? 0 : -EOPNOTSUPP;
+}
+
+void shadow_vcpu_init(struct vcpu *v)
+{
+    BUG_ON(!is_pv_domain(v->domain));
+    /* XXX v->arch.paging.mode = ???; */
+}
+
+void shadow_teardown(struct domain *d)
+{
+    BUG_ON(!is_pv_domain(d));
+}
+
+void shadow_final_teardown(struct domain *d)
+{
+    BUG_ON(!is_pv_domain(d));
+}
+
+int shadow_enable(struct domain *d, u32 mode)
+{
+    BUG_ON(!is_pv_domain(d));
+    return -EOPNOTSUPP;
+}
+
+void sh_remove_shadows(struct vcpu *v, mfn_t gmfn, int fast, int all)
+{
+}
+
+void shadow_blow_tables_per_domain(struct domain *d)
+{
+}
+
+int shadow_track_dirty_vram(struct domain *d,
+                            unsigned long begin_pfn, unsigned long nr,
+                            XEN_GUEST_HANDLE_64(uint8) dirty_bitmap)
+{
+    BUG();
+    return -EOPNOTSUPP;
+}
+
+int shadow_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
+                  XEN_GUEST_HANDLE_PARAM(void) u_domctl)
+{
+    return -EINVAL;
+}
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -88,6 +88,7 @@ void hypercall_page_initialise(struct do
 /*          shadow paging extension             */
 /************************************************/
 struct shadow_domain {
+#ifdef CONFIG_SHADOW_PAGING
     unsigned int      opt_flags;    /* runtime tunable optimizations on/off */
     struct page_list_head pinned_shadows;
 
@@ -117,9 +118,11 @@ struct shadow_domain {
 
     /* Has this domain ever used HVMOP_pagetable_dying? */
     bool_t pagetable_dying_op;
+#endif
 };
 
 struct shadow_vcpu {
+#ifdef CONFIG_SHADOW_PAGING
     /* PAE guests: per-vcpu shadow top-level table */
     l3_pgentry_t l3table[4] __attribute__((__aligned__(32)));
     /* PAE guests: per-vcpu cache of the top-level *guest* entries */
@@ -145,6 +148,7 @@ struct shadow_vcpu {
     } oos_fixup[SHADOW_OOS_PAGES];
 
     bool_t pagetable_dying;
+#endif
 };
 
 /************************************************/
--- a/xen/include/asm-x86/paging.h
+++ b/xen/include/asm-x86/paging.h
@@ -55,7 +55,11 @@
 #define PG_external    (XEN_DOMCTL_SHADOW_ENABLE_EXTERNAL << PG_mode_shift)
 
 #define paging_mode_enabled(_d)   ((_d)->arch.paging.mode)
+#ifdef CONFIG_SHADOW_PAGING
 #define paging_mode_shadow(_d)    ((_d)->arch.paging.mode & PG_SH_enable)
+#else
+#define paging_mode_shadow(_d)    ((void)(_d), 0)
+#endif
 #define paging_mode_hap(_d)       ((_d)->arch.paging.mode & PG_HAP_enable)
 
 #define paging_mode_refcounts(_d) ((_d)->arch.paging.mode & PG_refcounts)
@@ -74,6 +78,7 @@
 
 struct sh_emulate_ctxt;
 struct shadow_paging_mode {
+#ifdef CONFIG_SHADOW_PAGING
     void          (*detach_old_tables     )(struct vcpu *v);
     int           (*x86_emulate_write     )(struct vcpu *v, unsigned long va,
                                             void *src, u32 bytes,
@@ -88,6 +93,7 @@ struct shadow_paging_mode {
     int           (*guess_wrmap           )(struct vcpu *v, 
                                             unsigned long vaddr, mfn_t gmfn);
     void          (*pagetable_dying       )(struct vcpu *v, paddr_t gpa);
+#endif
     /* For outsiders to tell what mode we're in */
     unsigned int shadow_levels;
 };
--- a/xen/include/asm-x86/shadow.h
+++ b/xen/include/asm-x86/shadow.h
@@ -49,7 +49,7 @@
 
 /* Set up the shadow-specific parts of a domain struct at start of day.
  * Called from paging_domain_init(). */
-void shadow_domain_init(struct domain *d, unsigned int domcr_flags);
+int shadow_domain_init(struct domain *d, unsigned int domcr_flags);
 
 /* Setup the shadow-specific parts of a vcpu struct. It is called by
  * paging_vcpu_init() in paging.c */
--- a/xen/include/xen/paging.h
+++ b/xen/include/xen/paging.h
@@ -7,7 +7,7 @@
 #include <asm/paging.h>
 #include <asm/p2m.h>
 
-#elif defined CONFIG_SHADOW
+#elif defined CONFIG_SHADOW_PAGING
 
 #include <asm/shadow.h>