[PATCH v6 4/9] xen/arm: introduce put_page_nr and get_page_nr

[Penny Zheng <Penny.Zheng@xxxxxxx>]

Later, we need to add the right amount of references, which should be
the number of borrower domains, to the owner domain. Since we only have
get_page() to increment the page reference by 1, a loop is needed per
page, which is inefficient and time-consuming.

To save the loop time, this commit introduces a set of new helpers
put_page_nr() and get_page_nr() to increment/drop the page reference by nr.

Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
---
v6 change:
- no change
---
v5 change:
- no change
---
v4 changes:
- fix the assert about checking overflow to make sure that the right equation
return is at least equal to nr
- simplify the assert about checking the underflow
---
v3 changes:
- check overflow with "n"
- remove spurious change
- bring back the check that we enter the loop only when count_info is
greater than 0
---
v2 change:
- new commit
---
 xen/arch/arm/include/asm/mm.h |  4 ++++
 xen/arch/arm/mm.c             | 42 +++++++++++++++++++++++++++--------
 2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/xen/arch/arm/include/asm/mm.h b/xen/arch/arm/include/asm/mm.h
index c4bc3cd1e5..f1ab0fd030 100644
--- a/xen/arch/arm/include/asm/mm.h
+++ b/xen/arch/arm/include/asm/mm.h
@@ -343,6 +343,10 @@ void free_init_memory(void);
 int guest_physmap_mark_populate_on_demand(struct domain *d, unsigned long gfn,
                                           unsigned int order);
 
+extern bool get_page_nr(struct page_info *page, const struct domain *domain,
+                        unsigned long nr);
+extern void put_page_nr(struct page_info *page, unsigned long nr);
+
 extern void put_page_type(struct page_info *page);
 static inline void put_page_and_type(struct page_info *page)
 {
diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 009b8cd9ef..d654980706 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1587,21 +1587,29 @@ long arch_memory_op(int op, 
XEN_GUEST_HANDLE_PARAM(void) arg)
     return 0;
 }
 
-struct domain *page_get_owner_and_reference(struct page_info *page)
+static struct domain *page_get_owner_and_nr_reference(struct page_info *page,
+                                                      unsigned long nr)
 {
     unsigned long x, y = page->count_info;
     struct domain *owner;
 
+    /* Restrict nr to avoid "double" overflow */
+    if ( nr >= PGC_count_mask )
+    {
+        ASSERT_UNREACHABLE();
+        return NULL;
+    }
+
     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) )
+        if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
             return NULL;
     }
-    while ( (y = cmpxchg(&page->count_info, x, x + 1)) != x );
+    while ( (y = cmpxchg(&page->count_info, x, x + nr)) != x );
 
     owner = page_get_owner(page);
     ASSERT(owner);
@@ -1609,14 +1617,19 @@ struct domain *page_get_owner_and_reference(struct 
page_info *page)
     return owner;
 }
 
-void put_page(struct page_info *page)
+struct domain *page_get_owner_and_reference(struct page_info *page)
+{
+    return page_get_owner_and_nr_reference(page, 1);
+}
+
+void put_page_nr(struct page_info *page, unsigned long nr)
 {
     unsigned long nx, x, y = page->count_info;
 
     do {
-        ASSERT((y & PGC_count_mask) != 0);
+        ASSERT((y & PGC_count_mask) >= nr);
         x  = y;
-        nx = x - 1;
+        nx = x - nr;
     }
     while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
 
@@ -1626,19 +1639,30 @@ void put_page(struct page_info *page)
     }
 }
 
-bool get_page(struct page_info *page, const struct domain *domain)
+void put_page(struct page_info *page)
+{
+    put_page_nr(page, 1);
+}
+
+bool get_page_nr(struct page_info *page, const struct domain *domain,
+                 unsigned long nr)
 {
-    const struct domain *owner = page_get_owner_and_reference(page);
+    const struct domain *owner = page_get_owner_and_nr_reference(page, nr);
 
     if ( likely(owner == domain) )
         return true;
 
     if ( owner != NULL )
-        put_page(page);
+        put_page_nr(page, nr);
 
     return false;
 }
 
+bool get_page(struct page_info *page, const struct domain *domain)
+{
+    return get_page_nr(page, domain, 1);
+}
+
 /* 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
  * happy. */
-- 
2.25.1