Implement put_page(), as it will be used by  p2m_put_*-related code.

Although CONFIG_STATIC_MEMORY has not yet been introduced for RISC-V,
a stub for PGC_static is added to avoid cluttering the code of
put_page() with #ifdefs.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
---
Changes in V5:
 - Correct code style of do-while loop in put_page().
 - Add Acked-by: Jan Beulich <jbeulich@suse.com>.
---
Changes in V4:
 - Update the comment message:
   s/p2m_put_code/p2m_put_*-related code.
   s/put_page_nr/put_page.
---
 xen/arch/riscv/include/asm/mm.h |  7 +++++++
 xen/arch/riscv/mm.c             | 24 +++++++++++++++++++-----
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h
index dd8cdc9782..0503c92e6c 100644
--- a/xen/arch/riscv/include/asm/mm.h
+++ b/xen/arch/riscv/include/asm/mm.h
@@ -264,6 +264,13 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr)
 /* Page is Xen heap? */
 #define _PGC_xen_heap     PG_shift(2)
 #define PGC_xen_heap      PG_mask(1, 2)
+#ifdef CONFIG_STATIC_MEMORY
+/* Page is static memory */
+#define _PGC_static       PG_shift(3)
+#define PGC_static        PG_mask(1, 3)
+#else
+#define PGC_static     0
+#endif
 /* Page is broken? */
 #define _PGC_broken       PG_shift(7)
 #define PGC_broken        PG_mask(1, 7)
diff --git a/xen/arch/riscv/mm.c b/xen/arch/riscv/mm.c
index 1ef015f179..2e42293986 100644
--- a/xen/arch/riscv/mm.c
+++ b/xen/arch/riscv/mm.c
@@ -362,11 +362,6 @@ unsigned long __init calc_phys_offset(void)
     return phys_offset;
 }
 
-void put_page(struct page_info *page)
-{
-    BUG_ON("unimplemented");
-}
-
 void arch_dump_shared_mem_info(void)
 {
     BUG_ON("unimplemented");
@@ -627,3 +622,22 @@ void flush_page_to_ram(unsigned long mfn, bool sync_icache)
     if ( sync_icache )
         invalidate_icache();
 }
+
+void put_page(struct page_info *page)
+{
+    unsigned long nx, x, y = page->count_info;
+
+    do {
+        ASSERT((y & PGC_count_mask) >= 1);
+        x  = y;
+        nx = x - 1;
+    } while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
+
+    if ( unlikely((nx & PGC_count_mask) == 0) )
+    {
+        if ( unlikely(nx & PGC_static) )
+            free_domstatic_page(page);
+        else
+            free_domheap_page(page);
+    }
+}
-- 
2.51.0