Provide a RISC-V implementation of get_page_from_gfn(), matching the
semantics used by other architectures.

For translated guests, this is implemented as a wrapper around
p2m_get_page_from_gfn(). For DOMID_XEN, which is not auto-translated,
provide a 1:1 RAM/MMIO mapping and perform the required validation and
reference counting.

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>
---
Changes in v2:
 - Align implemntation with Arm's get_page_from_gfn().
 - Update the first comment about DOMID_XEN to mention that isn't "normal"
   domain instead of no-autotranslated.
 - Drop footer after commit message.
---
 xen/arch/riscv/include/asm/p2m.h |  8 ++------
 xen/arch/riscv/p2m.c             | 29 +++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/xen/arch/riscv/include/asm/p2m.h b/xen/arch/riscv/include/asm/p2m.h
index 60f27f9b347e..54ea67990f06 100644
--- a/xen/arch/riscv/include/asm/p2m.h
+++ b/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)
 {
diff --git a/xen/arch/riscv/p2m.c b/xen/arch/riscv/p2m.c
index 89e5db606fc8..11beaeead5ac 100644
--- a/xen/arch/riscv/p2m.c
+++ b/xen/arch/riscv/p2m.c
@@ -1534,3 +1534,32 @@ 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)
+{
+    struct page_info *page;
+    p2m_type_t p2mt;
+
+    /* Special case for DOMID_XEN as it isn't "normal" domain */
+    if ( likely(d != dom_xen) )
+        return p2m_get_page_from_gfn(p2m_get_hostp2m(d), _gfn(gfn), t);
+
+    if ( !t )
+        t = &p2mt;
+
+    *t = p2m_invalid;
+
+    /* DOMID_XEN sees 1-1 RAM. The p2m_type is based on the type of the page */
+    page = mfn_to_page(_mfn(gfn));
+
+    if ( !mfn_valid(_mfn(gfn)) || !get_page(page, d) )
+        return NULL;
+
+    if ( page->u.inuse.type_info & PGT_writable_page )
+        *t = p2m_ram_rw;
+    else
+        BUG_ON("unimplemented. p2m_ram_ro hasn't been introduced yet");
+
+    return page;
+}
-- 
2.53.0