[Xen-devel] [PATCH for-4.12 v2 13/17] xen/arm: p2m: Rework p2m_cache_flush_range

[Julien Grall <julien.grall@xxxxxxx>]

A follow-up patch will add support for preemption in p2m_cache_flush_range.
Because of the complexity for the 2 loops, it would be necessary to add
preemption in both of them.

This can be avoided by merging the 2 loops together and still keeping
the code fairly simple to read and extend.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    Changes in v2:
        - Patch added
---
 xen/arch/arm/p2m.c | 52 +++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 37 insertions(+), 15 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index c713226561..db22b53bfd 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1527,7 +1527,8 @@ int relinquish_p2m_mapping(struct domain *d)
 int p2m_cache_flush_range(struct domain *d, gfn_t start, gfn_t end)
 {
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
-    gfn_t next_gfn;
+    gfn_t next_block_gfn;
+    mfn_t mfn = INVALID_MFN;
     p2m_type_t t;
     unsigned int order;
 
@@ -1542,24 +1543,45 @@ int p2m_cache_flush_range(struct domain *d, gfn_t 
start, gfn_t end)
     start = gfn_max(start, p2m->lowest_mapped_gfn);
     end = gfn_min(end, p2m->max_mapped_gfn);
 
-    for ( ; gfn_x(start) < gfn_x(end); start = next_gfn )
-    {
-        mfn_t mfn = p2m_get_entry(p2m, start, &t, NULL, &order, NULL);
+    next_block_gfn = start;
 
-        next_gfn = gfn_next_boundary(start, order);
-
-        /* Skip hole and non-RAM page */
-        if ( mfn_eq(mfn, INVALID_MFN) || !p2m_is_any_ram(t) )
-            continue;
-
-        /* XXX: Implement preemption */
-        while ( gfn_x(start) < gfn_x(next_gfn) )
+    while ( gfn_x(start) < gfn_x(end) )
+    {
+        /*
+         * We want to flush page by page as:
+         *  - it may not be possible to map the full block (can be up to 1GB)
+         *    in Xen memory
+         *  - we may want to do fine grain preemption as flushing multiple
+         *    page in one go may take a long time
+         *
+         * As p2m_get_entry is able to return the size of the mapping
+         * in the p2m, it is pointless to execute it for each page.
+         *
+         * We can optimize it by tracking the gfn of the next
+         * block. So we will only call p2m_get_entry for each block (can
+         * be up to 1GB).
+         */
+        if ( gfn_eq(start, next_block_gfn) )
         {
-            flush_page_to_ram(mfn_x(mfn), false);
+            mfn = p2m_get_entry(p2m, start, &t, NULL, &order, NULL);
+            next_block_gfn = gfn_next_boundary(start, order);
 
-            start = gfn_add(start, 1);
-            mfn = mfn_add(mfn, 1);
+            /*
+             * The following regions can be skipped:
+             *      - Hole
+             *      - non-RAM
+             */
+            if ( mfn_eq(mfn, INVALID_MFN) || !p2m_is_any_ram(t) )
+            {
+                start = next_block_gfn;
+                continue;
+            }
         }
+
+        flush_page_to_ram(mfn_x(mfn), false);
+
+        start = gfn_add(start, 1);
+        mfn = mfn_add(mfn, 1);
     }
 
     invalidate_icache();
-- 
2.11.0


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