[xen master] x86/shadow: VRAM last_dirty tagging

[patchbot@xxxxxxx]

commit 2965f75ebd3814158878464d3c15c3b94fb96571
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu May 21 09:10:58 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu May 21 09:10:58 2026 +0200

    x86/shadow: VRAM last_dirty tagging
    
    ->last_dirty holding a valid value (one other than -1) is solely an
    indication of the bitmap being entirely clean. (The opposite isn't true,
    because of _sh_propagate() setting the field to a valid value without
    setting a bit in the bitmap.) As a consequence
    - setting the field to a valid value right after having allocated zero-
      filled space is pointless,
    - copying the all empty bitmap to the output array is pointless; with the
      output array also having been allocated zero-filled, not even a memset()
      is needed there,
    - after restoring bitmap contents when dealing with copy_to_guest() having
      failed, the field needs setting to a valid value again.
    
    Furthermore invoking NOW() in perhaps many loop iterations of the main
    loop is wasteful, too. Record whether any bit was set, and record a new
    ->last_dirty only once, after the loop. Then use the same NOW() value also
    for the subsequent check.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm/shadow/hvm.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/hvm.c b/xen/arch/x86/mm/shadow/hvm.c
index cf4862cbc9..ff4dd1f94c 100644
--- a/xen/arch/x86/mm/shadow/hvm.c
+++ b/xen/arch/x86/mm/shadow/hvm.c
@@ -1068,18 +1068,18 @@ int shadow_track_dirty_vram(struct domain *d,
         if ( (dirty_vram->dirty_bitmap = xzalloc_array(uint8_t, dirty_size)) 
== NULL )
             goto out_sl1ma;
 
-        dirty_vram->last_dirty = NOW();
+        dirty_vram->last_dirty = -1;
 
         /* Tell the caller that this time we could not track dirty bits. */
         rc = -ENODATA;
     }
-    else if ( dirty_vram->last_dirty == -1 )
-        /* still completely clean, just copy our empty bitmap */
-        memcpy(dirty_bitmap, dirty_vram->dirty_bitmap, dirty_size);
-    else
+    /* Nothing to do when the bitmap is still completely clean. */
+    else if ( dirty_vram->last_dirty != -1 )
     {
         mfn_t map_mfn = INVALID_MFN;
         void *map_sl1p = NULL;
+        bool any_dirty = false;
+        s_time_t now;
 
         /* Iterate over VRAM to track dirty bits. */
         for ( i = 0; i < nr_frames; i++ )
@@ -1155,16 +1155,20 @@ int shadow_track_dirty_vram(struct domain *d,
             if ( dirty )
             {
                 dirty_vram->dirty_bitmap[i / 8] |= 1 << (i % 8);
-                dirty_vram->last_dirty = NOW();
+                any_dirty = true;
             }
         }
 
+        now = NOW();
+        if ( any_dirty )
+            dirty_vram->last_dirty = now;
+
         if ( map_sl1p )
             unmap_domain_page(map_sl1p);
 
         memcpy(dirty_bitmap, dirty_vram->dirty_bitmap, dirty_size);
         memset(dirty_vram->dirty_bitmap, 0, dirty_size);
-        if ( dirty_vram->last_dirty + SECONDS(2) < NOW() )
+        if ( dirty_vram->last_dirty + SECONDS(2) < now )
         {
             /*
              * Was clean for more than two seconds, try to disable guest
@@ -1197,6 +1201,7 @@ int shadow_track_dirty_vram(struct domain *d,
         paging_lock(d);
         for ( i = 0; i < dirty_size; i++ )
             dirty_vram->dirty_bitmap[i] |= dirty_bitmap[i];
+        dirty_vram->last_dirty = NOW();
         paging_unlock(d);
         rc = -EFAULT;
     }
--
generated by git-patchbot for /home/xen/git/xen.git#master