[xen staging] tools/libs/guest: Use superpages where possible on migrate/resume

[patchbot@xxxxxxx]

commit 50baf2d9f7bb8502d497385846789ea1755901cc
Author:     Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
AuthorDate: Fri Sep 12 10:57:44 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Sep 29 21:32:37 2025 +0200

    tools/libs/guest: Use superpages where possible on migrate/resume
    
    Right now on migrate/resume, all guest memory is allocated with 4k
    small pages.  This was an intended oversight in the original Migration
    v2 implementation.
    
    With some test memory program stressing TLB (many small random memory
    accesses) superpages can show a 15% performance improvement.
    
    On the first memory iteration the sender is currently sending memory
    in 4MB aligned chunks of GFN space which allows the receiver to
    allocate most pages as 2MB superpages instead of single 4KB pages.
    
    This change does not handle 1GB superpages as this will require
    changes to the migration protocol.
    
    Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 tools/libs/guest/xg_sr_restore.c | 57 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 54 insertions(+), 3 deletions(-)

diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c
index 06231ca826..4aed5cf4ac 100644
--- a/tools/libs/guest/xg_sr_restore.c
+++ b/tools/libs/guest/xg_sr_restore.c
@@ -129,6 +129,35 @@ static int pfn_set_populated(struct xc_sr_context *ctx, 
xen_pfn_t pfn)
     return 0;
 }
 
+#if defined(__i386__) || defined(__x86_64__)
+/* Order of the smallest superpage */
+#define SMALL_SUPERPAGE_ORDER 9
+#else
+#error Define SMALL_SUPERPAGE_ORDER for this platform
+#endif
+
+static bool populate_small_superpage(struct xc_sr_context *ctx, xen_pfn_t gfn)
+{
+    xen_pfn_t mfn = gfn;
+
+    if ( xc_domain_populate_physmap_exact(
+         ctx->xch, ctx->domid, 1, SMALL_SUPERPAGE_ORDER, 0, &mfn) )
+        return false;
+
+    /*
+     * XENMEM_populate_physmap has no coherent error semantics.
+     * Assume a failure here is ENOMEM, and fall back to allocating
+     * small pages.
+     */
+    if ( mfn == INVALID_MFN )
+        return false;
+
+    for ( size_t i = 0; i < (1UL << SMALL_SUPERPAGE_ORDER); ++i )
+        ctx->restore.ops.set_gfn(ctx, gfn + i, mfn + i);
+
+    return true;
+}
+
 /*
  * Given a set of pfns, obtain memory from Xen to fill the physmap for the
  * unpopulated subset.  If types is NULL, no page type checking is performed
@@ -142,6 +171,9 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int 
count,
         *pfns = malloc(count * sizeof(*pfns));
     unsigned int i, nr_pfns = 0;
     int rc = -1;
+    xen_pfn_t prev = 0;
+    unsigned int num_contiguous = 0;
+    xen_pfn_t mask = (1ULL << SMALL_SUPERPAGE_ORDER) - 1;
 
     if ( !mfns || !pfns )
     {
@@ -152,14 +184,33 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int 
count,
 
     for ( i = 0; i < count; ++i )
     {
+        xen_pfn_t pfn = original_pfns[i];
+
         if ( (!types || page_type_to_populate(types[i])) &&
-             !pfn_is_populated(ctx, original_pfns[i]) )
+             !pfn_is_populated(ctx, pfn) )
         {
-            rc = pfn_set_populated(ctx, original_pfns[i]);
+            rc = pfn_set_populated(ctx, pfn);
             if ( rc )
                 goto err;
-            pfns[nr_pfns] = mfns[nr_pfns] = original_pfns[i];
+            pfns[nr_pfns] = mfns[nr_pfns] = pfn;
             ++nr_pfns;
+
+            /*
+             * During the first pass for x86 HVM guests, PAGE_DATA
+             * records contain metadata about 4M aligned chunks of GFN
+             * space.  Reconstruct 2M superpages where possible.
+             */
+            if ( pfn != prev + 1 )
+                num_contiguous = 0;
+            num_contiguous++;
+            prev = pfn;
+
+            if ( num_contiguous > mask && (pfn & mask) == mask &&
+                 populate_small_superpage(ctx, pfn - mask) )
+            {
+                nr_pfns -= mask + 1;
+                num_contiguous = 0;
+            }
         }
     }
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging