[PATCH v1 1/6] xen/riscv: implement get_page_from_gfn()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

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@xxxxxxxxx>
---
Does it make sense to make this function almost fully generic?

It looks like most of the logic here is architecture-independent and identical
across architectures, except for the following points:

1. ```
   if ( likely(d != dom_xen) )
   ```

   This could be made generic by introducing paging_mode_translate() for ARM
   and defining it as `(d != dom_xen)` there.

2. ```
   if ( t )
       *t = likely(d != dom_io) ? p2m_ram_rw : p2m_mmio_direct_io;
   ```

   Here, only `p2m_mmio_direct_io` appears to be architecture-specific. This
   could be abstracted via a helper such as `dom_io_p2m_type()` and used here
   instead.
---
---
 xen/arch/riscv/include/asm/p2m.h |  8 ++------
 xen/arch/riscv/p2m.c             | 28 ++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/xen/arch/riscv/include/asm/p2m.h b/xen/arch/riscv/include/asm/p2m.h
index 0cdd3dc44683..c68494593fd9 100644
--- a/xen/arch/riscv/include/asm/p2m.h
+++ b/xen/arch/riscv/include/asm/p2m.h
@@ -171,12 +171,8 @@ 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 275c38020ae2..f5b03e1e3264 100644
--- a/xen/arch/riscv/p2m.c
+++ b/xen/arch/riscv/p2m.c
@@ -1557,3 +1557,31 @@ void p2m_handle_vmenter(void)
         flush_tlb_guest_local();
     }
 }
+
+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;
+
+    /*
+     * Special case for DOMID_XEN as it is the only domain so far that is
+     * not auto-translated.
+     */
+    if ( likely(d != dom_xen) )
+        return p2m_get_page_from_gfn(p2m_get_hostp2m(d), _gfn(gfn), t);
+
+    /* Non-translated guests see 1-1 RAM / MMIO mappings everywhere */
+
+    if ( t )
+        *t = p2m_invalid;
+
+    page = mfn_to_page(_mfn(gfn));
+
+    if ( !mfn_valid(_mfn(gfn)) || !get_page(page, d) )
+        return NULL;
+
+    if ( t )
+        *t = likely(d != dom_io) ? p2m_ram_rw : p2m_mmio_direct_io;
+
+    return page;
+}
-- 
2.52.0