Implement the mfn_valid() macro to verify whether a given MFN is valid by
checking that it falls within the range [start_page, max_page).
These bounds are initialized based on the start and end addresses of RAM.

As part of this patch, start_page is introduced and initialized with the
PFN of the first RAM page.
Also, initialize pdx_group_valid() by calling set_pdx_range() when
memory banks are being mapped.

Also, after providing a non-stub implementation of the mfn_valid() macro,
the following compilation errors started to occur:
  riscv64-linux-gnu-ld: prelink.o: in function `alloc_heap_pages':
  /build/xen/common/page_alloc.c:1054: undefined reference to `page_is_offlinable'
  riscv64-linux-gnu-ld: /build/xen/common/page_alloc.c:1035: undefined reference to `page_is_offlinable'
  riscv64-linux-gnu-ld: prelink.o: in function `reserve_offlined_page':
  /build/xen/common/page_alloc.c:1151: undefined reference to `page_is_offlinable'
  riscv64-linux-gnu-ld: ./.xen-syms.0: hidden symbol `page_is_offlinable' isn't defined
  riscv64-linux-gnu-ld: final link failed: bad value
  make[2]: *** [arch/riscv/Makefile:28: xen-syms] Error 1

To resolve these errors, the following functions have also been introduced,
based on their Arm counterparts:
- page_get_owner_and_reference() and its variant to safely acquire a
  reference to a page and retrieve its owner.
- Implement page_is_offlinable() to return false for RISC-V.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
---
Changes in V5:
- Move declaration/defintion of page_is_offlinale() before put_page() to have
  get_ and put_ functions together.
- Correct code style of do-while loop.
- Add Acked-by: Jan Beulich <jbeulich@suse.com>.
---
Changes in V4:
 - Rebase the patch on top of patch series "[PATCH v2 0/2] constrain page_is_ram_type() to x86".
 - Add implementation of page_is_offlinable() instead of page_is_ram().
 - Update the commit message.
---
Changes in V3:
 - Update defintion of mfn_valid().
 - Use __ro_after_init for variable start_page.
 - Drop ASSERT_UNREACHABLE() in page_get_owner_and_nr_reference().
 - Update the comment inside do/while in page_get_owner_and_nr_reference().
 - Define _PGC_static and drop "#ifdef CONFIG_STATIC_MEMORY" in put_page_nr().
 - Initialize pdx_group_valid() by calling set_pdx_range() when memory banks are mapped.
 - Drop page_get_owner_and_nr_reference() and implement page_get_owner_and_reference()
   without reusing of a page_get_owner_and_nr_reference() to avoid potential dead code.
 - Move defintion of get_page() to "xen/riscv: add support of page lookup by GFN", where
   it is really used.
---
Changes in V2:
 - New patch.
---
 xen/arch/riscv/include/asm/mm.h |  9 +++++++--
 xen/arch/riscv/mm.c             | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h
index 0503c92e6c..1b16809749 100644
--- a/xen/arch/riscv/include/asm/mm.h
+++ b/xen/arch/riscv/include/asm/mm.h
@@ -5,6 +5,7 @@
 
 #include <public/xen.h>
 #include <xen/bug.h>
+#include <xen/compiler.h>
 #include <xen/const.h>
 #include <xen/mm-frame.h>
 #include <xen/pdx.h>
@@ -300,8 +301,12 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr)
 #define page_get_owner(p)    (p)->v.inuse.domain
 #define page_set_owner(p, d) ((p)->v.inuse.domain = (d))
 
-/* TODO: implement */
-#define mfn_valid(mfn) ({ (void)(mfn); 0; })
+extern unsigned long start_page;
+
+#define mfn_valid(mfn) ({                                               \
+    unsigned long tmp_mfn = mfn_x(mfn);                                 \
+    likely((tmp_mfn >= start_page)) && likely(__mfn_valid(tmp_mfn));    \
+})
 
 #define domain_set_alloc_bitsize(d) ((void)(d))
 #define domain_clamp_alloc_bitsize(d, b) ((void)(d), (b))
diff --git a/xen/arch/riscv/mm.c b/xen/arch/riscv/mm.c
index 2e42293986..e25f995b72 100644
--- a/xen/arch/riscv/mm.c
+++ b/xen/arch/riscv/mm.c
@@ -521,6 +521,8 @@ static void __init setup_directmap_mappings(unsigned long base_mfn,
 #error setup_{directmap,frametable}_mapping() should be implemented for RV_32
 #endif
 
+unsigned long __ro_after_init start_page;
+
 /*
  * Setup memory management
  *
@@ -570,9 +572,13 @@ void __init setup_mm(void)
         ram_end = max(ram_end, bank_end);
 
         setup_directmap_mappings(PFN_DOWN(bank_start), PFN_DOWN(bank_size));
+
+        set_pdx_range(paddr_to_pfn(bank_start), paddr_to_pfn(bank_end));
     }
 
     setup_frametable_mappings(ram_start, ram_end);
+
+    start_page = PFN_DOWN(ram_start);
     max_page = PFN_DOWN(ram_end);
 }
 
@@ -623,6 +629,11 @@ void flush_page_to_ram(unsigned long mfn, bool sync_icache)
         invalidate_icache();
 }
 
+bool page_is_offlinable(mfn_t mfn)
+{
+    return false;
+}
+
 void put_page(struct page_info *page)
 {
     unsigned long nx, x, y = page->count_info;
@@ -641,3 +652,24 @@ void put_page(struct page_info *page)
             free_domheap_page(page);
     }
 }
+
+struct domain *page_get_owner_and_reference(struct page_info *page)
+{
+    unsigned long x, y = page->count_info;
+    struct domain *owner;
+
+    do {
+        x = y;
+        /*
+         * Count ==  0: Page is not allocated, so we cannot take a reference.
+         * Count == -1: Reference count would wrap, which is invalid.
+         */
+        if ( unlikely(((x + 1) & PGC_count_mask) <= 1) )
+            return NULL;
+    } while ( (y = cmpxchg(&page->count_info, x, x + 1)) != x );
+
+    owner = page_get_owner(page);
+    ASSERT(owner);
+
+    return owner;
+}
-- 
2.51.0