[Xen-devel] [RFC 07/16] xen/arm: p2m: Introduce p2m_is_valid and use it

[Julien Grall <julien.grall@xxxxxxx>]

The LPAE format allows to store information in an entry even with the
valid bit unset. In a follow-up patch, we will take advantage of this
feature to re-purpose the valid bit for generating a translation fault
even if an entry contains valid information.

So we need a different way to know whether an entry contains valid
information. It is possible to use the information hold in the p2m_type
to know for that purpose. Indeed all entries containing valid
information will have a valid p2m type (i.e p2m_type != p2m_invalid).

This patch introduces a new helper p2m_is_valid, which implements that
idea, and replace most of lpae_is_valid call with the new helper. The ones
remaining are for TLBs handling and entries accounting.

With the renaming there are 2 others changes required:
    - Generate table entry with a valid p2m type
    - Detect new mapping for proper stats accounting

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
---
 xen/arch/arm/p2m.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 6c76298ebc..2a1e7e9be2 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -220,17 +220,26 @@ static p2m_access_t p2m_mem_access_radix_get(struct 
p2m_domain *p2m, gfn_t gfn)
 }
 
 /*
+ * In the case of the P2M, the valid bit is used for other purpose. Use
+ * the type to check whether an entry is valid.
+ */
+static inline bool p2m_is_valid(lpae_t pte)
+{
+    return pte.p2m.type != p2m_invalid;
+}
+
+/*
  * lpae_is_* helpers don't check whether the valid bit is set in the
  * PTE. Provide our own overlay to check the valid bit.
  */
 static inline bool p2m_is_mapping(lpae_t pte, unsigned int level)
 {
-    return lpae_is_valid(pte) && lpae_is_mapping(pte, level);
+    return p2m_is_valid(pte) && lpae_is_mapping(pte, level);
 }
 
 static inline bool p2m_is_superpage(lpae_t pte, unsigned int level)
 {
-    return lpae_is_valid(pte) && lpae_is_superpage(pte, level);
+    return p2m_is_valid(pte) && lpae_is_superpage(pte, level);
 }
 
 #define GUEST_TABLE_MAP_FAILED 0
@@ -264,7 +273,7 @@ static int p2m_next_level(struct p2m_domain *p2m, bool 
read_only,
 
     entry = *table + offset;
 
-    if ( !lpae_is_valid(*entry) )
+    if ( !p2m_is_valid(*entry) )
     {
         if ( read_only )
             return GUEST_TABLE_MAP_FAILED;
@@ -356,7 +365,7 @@ mfn_t p2m_get_entry(struct p2m_domain *p2m, gfn_t gfn,
 
     entry = table[offsets[level]];
 
-    if ( lpae_is_valid(entry) )
+    if ( p2m_is_valid(entry) )
     {
         *t = entry.p2m.type;
 
@@ -544,8 +553,11 @@ static lpae_t page_to_p2m_table(struct page_info *page)
     /*
      * The access value does not matter because the hardware will ignore
      * the permission fields for table entry.
+     *
+     * We use p2m_ram_rw so the entry has a valid type. This is important
+     * for p2m_is_valid() to return valid on table entries.
      */
-    return mfn_to_p2m_entry(page_to_mfn(page), p2m_invalid, p2m_access_rwx);
+    return mfn_to_p2m_entry(page_to_mfn(page), p2m_ram_rw, p2m_access_rwx);
 }
 
 static inline void p2m_write_pte(lpae_t *p, lpae_t pte, bool clean_pte)
@@ -569,7 +581,7 @@ static int p2m_create_table(struct p2m_domain *p2m, lpae_t 
*entry)
     struct page_info *page;
     lpae_t *p;
 
-    ASSERT(!lpae_is_valid(*entry));
+    ASSERT(!p2m_is_valid(*entry));
 
     page = alloc_domheap_page(NULL, 0);
     if ( page == NULL )
@@ -626,7 +638,7 @@ static int p2m_mem_access_radix_set(struct p2m_domain *p2m, 
gfn_t gfn,
  */
 static void p2m_put_l3_page(const lpae_t pte)
 {
-    ASSERT(lpae_is_valid(pte));
+    ASSERT(p2m_is_valid(pte));
 
     /*
      * TODO: Handle other p2m types
@@ -654,11 +666,11 @@ static void p2m_free_entry(struct p2m_domain *p2m,
     struct page_info *pg;
 
     /* Nothing to do if the entry is invalid. */
-    if ( !lpae_is_valid(entry) )
+    if ( !p2m_is_valid(entry) )
         return;
 
     /* Nothing to do but updating the stats if the entry is a super-page. */
-    if ( p2m_is_superpage(entry, level) )
+    if ( level == 3 && entry.p2m.table )
     {
         p2m->stats.mappings[level]--;
         return;
@@ -951,7 +963,7 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
             else
                 p2m->need_flush = true;
         }
-        else /* new mapping */
+        else if ( !p2m_is_valid(orig_pte) ) /* new mapping */
             p2m->stats.mappings[level]++;
 
         p2m_write_pte(entry, pte, p2m->clean_pte);
@@ -965,7 +977,7 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
      * Free the entry only if the original pte was valid and the base
      * is different (to avoid freeing when permission is changed).
      */
-    if ( lpae_is_valid(orig_pte) &&
+    if ( p2m_is_valid(orig_pte) &&
          !mfn_eq(lpae_get_mfn(*entry), lpae_get_mfn(orig_pte)) )
         p2m_free_entry(p2m, orig_pte, level);
 
-- 
2.11.0


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