[PATCH v2 for 4.20? 1/3] xen/riscv: implement software page table walking

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

RISC-V doesn't have hardware feature to ask MMU to translate
virtual address to physical address ( like Arm has, for example ),
so software page table walking is implemented.

As pte_walk() returns pte_t and it requires inclusion of <asm/page.h>
in <asm/mm.h>, the following compilation started to occur after this
inclusion:
- implicit declaration of function 'GENMASK'. To resolve this, <xen/bitops.h>
  needs to be included at the top of <asm/cmpxchg.h>.
- implicit declaration of map_domain_page() and unmap_domain_page(). This issue
  arises because, after including <asm/page.h> in <asm/mm.h>, we could have the
  following hierarchy if someone decides to include <xen/domain.h>:
    <xen/domain_page.h>
      <xen/mm.h>
        <asm/mm.h>
          <asm/page.h>
            <xen/domain_page.h>
            ...
            flush_to_ram() which uses {un}map_domain_page() <---
            ...
    Declaration of {un}map_domain_page() happens here <---

    To avoid this compilation issue, definition of flush_to_ram() is moved to
    mm.c.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in v2:
 - Update the code of pt_walk() to return pte_t instead of paddr_t.
 - Fix typos and drop blankets inside parentheses in the comment.
 - Update the `ret` handling; there is no need for `mfn` calculation
   anymore as pt_walk() returns or pte_t of a leaf node or non-present
   pte_t.
 - Drop the comment before unmap_table().
 - Drop local variable `pa` as pt_walk() is going to return pte_t
   instead of paddr_t.
 - Add the comment above pt_walk() to explain what it does and returns.
 - Add additional explanation about possible return values of pt_next_level()
   used inside while loop in pt_walk().
 - Change `pa` to `table` in the comment before while loop in pt_walk()
   as actually this loop finds a page table where paga table entry for `va`
   is located.
 - After including <asm/page.h> in <asm/mm.h>, the following compilation
   error occurs:
    ./arch/riscv/include/asm/cmpxchg.h:56:9: error: implicit declaration of
    function 'GENMASK'
   To resolve this, <xen/bitops.h> needs to be included at the top of
   <asm/cmpxchg.h>.
 - To avoid an issue with the implicit declaration of map_domain_page() and
   unmap_domain_page() after including <asm/page.h> in <asm/mm.h>, the
   implementation of flush_page_to_ram() has been moved to mm.c. (look for
   more detailed explanation in the commit message) As a result drop
   inclusion of <xen/domain.h> in <asm/page.h>.
 - Update the commit message
---
 xen/arch/riscv/include/asm/cmpxchg.h |  1 +
 xen/arch/riscv/include/asm/mm.h      |  4 ++
 xen/arch/riscv/include/asm/page.h    | 14 +------
 xen/arch/riscv/mm.c                  | 14 +++++++
 xen/arch/riscv/pt.c                  | 55 ++++++++++++++++++++++++++++
 5 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/xen/arch/riscv/include/asm/cmpxchg.h 
b/xen/arch/riscv/include/asm/cmpxchg.h
index 662d3fd5d4..4cfe5927b7 100644
--- a/xen/arch/riscv/include/asm/cmpxchg.h
+++ b/xen/arch/riscv/include/asm/cmpxchg.h
@@ -4,6 +4,7 @@
 #ifndef ASM__RISCV__CMPXCHG_H
 #define ASM__RISCV__CMPXCHG_H
 
+#include <xen/bitops.h>
 #include <xen/compiler.h>
 #include <xen/lib.h>
 
diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h
index 292aa48fc1..10a15a8b03 100644
--- a/xen/arch/riscv/include/asm/mm.h
+++ b/xen/arch/riscv/include/asm/mm.h
@@ -156,6 +156,10 @@ static inline struct page_info *virt_to_page(const void *v)
     return frametable_virt_start + PFN_DOWN(va - directmap_virt_start);
 }
 
+#include <asm/page.h>
+
+pte_t * pt_walk(vaddr_t va, unsigned int *pte_level);
+
 /*
  * Common code requires get_page_type and put_page_type.
  * We don't care about typecounts so we just do the minimum to make it
diff --git a/xen/arch/riscv/include/asm/page.h 
b/xen/arch/riscv/include/asm/page.h
index 7a6174a109..b9076173f4 100644
--- a/xen/arch/riscv/include/asm/page.h
+++ b/xen/arch/riscv/include/asm/page.h
@@ -7,7 +7,6 @@
 
 #include <xen/bug.h>
 #include <xen/const.h>
-#include <xen/domain_page.h>
 #include <xen/errno.h>
 #include <xen/types.h>
 
@@ -177,18 +176,7 @@ static inline void invalidate_icache(void)
 #define clear_page(page) memset((void *)(page), 0, PAGE_SIZE)
 #define copy_page(dp, sp) memcpy(dp, sp, PAGE_SIZE)
 
-static inline void flush_page_to_ram(unsigned long mfn, bool sync_icache)
-{
-    const void *v = map_domain_page(_mfn(mfn));
-
-    if ( clean_and_invalidate_dcache_va_range(v, PAGE_SIZE) )
-        BUG();
-
-    unmap_domain_page(v);
-
-    if ( sync_icache )
-        invalidate_icache();
-}
+void flush_page_to_ram(unsigned long mfn, bool sync_icache);
 
 /* Write a pagetable entry. */
 static inline void write_pte(pte_t *p, pte_t pte)
diff --git a/xen/arch/riscv/mm.c b/xen/arch/riscv/mm.c
index f2bf279bac..32574c879b 100644
--- a/xen/arch/riscv/mm.c
+++ b/xen/arch/riscv/mm.c
@@ -3,6 +3,7 @@
 #include <xen/bootfdt.h>
 #include <xen/bug.h>
 #include <xen/compiler.h>
+#include <xen/domain_page.h>
 #include <xen/init.h>
 #include <xen/kernel.h>
 #include <xen/libfdt/libfdt.h>
@@ -564,3 +565,16 @@ void *__init arch_vmap_virt_end(void)
 {
     return (void *)(VMAP_VIRT_START + VMAP_VIRT_SIZE);
 }
+
+void flush_page_to_ram(unsigned long mfn, bool sync_icache)
+{
+    const void *v = map_domain_page(_mfn(mfn));
+
+    if ( clean_and_invalidate_dcache_va_range(v, PAGE_SIZE) )
+        BUG();
+
+    unmap_domain_page(v);
+
+    if ( sync_icache )
+        invalidate_icache();
+}
diff --git a/xen/arch/riscv/pt.c b/xen/arch/riscv/pt.c
index a703e0f1bd..2a5a191a70 100644
--- a/xen/arch/riscv/pt.c
+++ b/xen/arch/riscv/pt.c
@@ -274,6 +274,61 @@ static int pt_update_entry(mfn_t root, vaddr_t virt,
     return rc;
 }
 
+/*
+ * pt_walk() performs software page table walking and returns the pte_t of
+ * a leaf node or the leaf-most not-present pte_t if no leaf node is found
+ * for further analysis.
+ * Additionally, pt_walk() returns the level of the found pte.
+ */
+pte_t * pt_walk(vaddr_t va, unsigned int *pte_level)
+{
+    const mfn_t root = get_root_page();
+    /*
+     * In pt_walk() only XEN_TABLE_MAP_NONE and XEN_TABLE_SUPER_PAGE are
+     * handled (as they are only possible for page table walking), so
+     * initialize `ret` with "impossible" XEN_TABLE_MAP_NOMEM.
+     */
+    int ret = XEN_TABLE_MAP_NOMEM;
+    unsigned int level = HYP_PT_ROOT_LEVEL;
+    pte_t *table;
+
+    DECLARE_OFFSETS(offsets, va);
+
+    table = map_table(root);
+
+    /*
+     * Find `table` of an entry which corresponds to `va` by iterating for each
+     * page level and checking if the entry points to a next page table or
+     * to a page.
+     *
+     * Two cases are possible:
+     * - ret == XEN_TABLE_SUPER_PAGE means that the entry was find;
+     *   (Despite the name) XEN_TABLE_SUPER_PAGE also covers 4K mappings. If
+     *   pt_next_level() is called for page table level 0, it results in the
+     *   entry being a pointer to a leaf node, thereby returning
+     *   XEN_TABLE_SUPER_PAGE, despite of the fact this leaf covers 4k mapping.
+     * - ret == XEN_TABLE_MAP_NONE means that requested `va` wasn't actually
+     *   mapped.
+     */
+    while ( (ret != XEN_TABLE_MAP_NONE) && (ret != XEN_TABLE_SUPER_PAGE) )
+    {
+        /*
+         * This case shouldn't really occur as it will mean that for table
+         * level 0 a pointer to next page table has been written, but at
+         * level 0 it could be only a pointer to 4k page.
+         */
+        ASSERT(level <= HYP_PT_ROOT_LEVEL);
+
+        ret = pt_next_level(false, &table, offsets[level]);
+        level--;
+    }
+
+    if ( pte_level )
+        *pte_level = level + 1;
+
+    return table + offsets[level + 1];
+}
+
 /* Return the level where mapping should be done */
 static int pt_mapping_level(unsigned long vfn, mfn_t mfn, unsigned long nr,
                             unsigned int flags)
-- 
2.48.1