xen/riscv: implement get_page_from_gfn()

The function is implemented out-of-line rather than as a static inline,
to avoid header ordering issues where struct domain is incomplete when
asm/p2m.h is included, leading to build failures:
  In file included from ./arch/riscv/include/asm/domain.h:10,
                   from ./include/xen/domain.h:16,
                   from ./include/xen/sched.h:11,
                   from ./include/xen/event.h:12,
                   from common/cpu.c:3:
  ./arch/riscv/include/asm/p2m.h: In function 'get_page_from_gfn':
  ./arch/riscv/include/asm/p2m.h:50:33: error: invalid use of undefined type 'struct domain'
     50 | #define p2m_get_hostp2m(d) (&(d)->arch.p2m)
        |                                 ^~
  ./arch/riscv/include/asm/p2m.h:180:38: note: in expansion of macro 'p2m_get_hostp2m'
    180 |         return p2m_get_page_from_gfn(p2m_get_hostp2m(d), _gfn(gfn), t);
        |                                      ^~~~~~~~~~~~~~~
  make[2]: *** [Rules.mk:253: common/cpu.o] Error 1
  make[1]: *** [build.mk:72: common] Error 2
  make: *** [Makefile:623: xen] Error 2

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/p2m.h

@@ -164,12 +164,8 @@ typedef unsigned int p2m_query_t;
 #define P2M_ALLOC    (1u<<0)   /* Populate PoD and paged-out entries */
 #define P2M_UNSHARE  (1u<<1)   /* Break CoW sharing */
 
-static inline struct page_info *get_page_from_gfn(
-    struct domain *d, unsigned long gfn, p2m_type_t *t, p2m_query_t q)
-{
-    BUG_ON("unimplemented");
-    return NULL;
-}
+struct page_info *get_page_from_gfn(struct domain *d, unsigned long gfn,
+                                    p2m_type_t *t, p2m_query_t q);
 
 static inline void memory_type_changed(struct domain *d)
 {

xen/arch/riscv/p2m.c

@@ -1534,3 +1534,16 @@ void p2m_handle_vmenter(void)
      * won't be reused until need_flush is set to true.
      */
 }
+
+struct page_info *get_page_from_gfn(struct domain *d, unsigned long gfn,
+                                    p2m_type_t *t, p2m_query_t q)
+{
+    /*
+     * System domains are domains which don't have p2m translation tables,
+     * so they can't use p2m_get_page_from_gfn() and extra care should be
+     * taken for them.
+     */
+    ASSERT(!is_system_domain(d));
+
+    return p2m_get_page_from_gfn(p2m_get_hostp2m(d), _gfn(gfn), t);
+}