[Xen-devel] [PATCH] x86/XPTI: fix S3 resume (and CPU offlining in general)

["Jan Beulich" <JBeulich@xxxxxxxx>]

In commit d1d6fc97d6 ("x86/xpti: really hide almost all of Xen image")
I've failed to remember the fact that multiple CPUs share a stub
mapping page. Therefore it is wrong to unconditionally zap the mapping
when bringing down a CPU; it may only be unmapped when no other online
CPU uses that same page.

Reported-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -876,7 +876,21 @@ static void cleanup_cpu_root_pgt(unsigne
 
     free_xen_pagetable(rpt);
 
-    /* Also zap the stub mapping for this CPU. */
+    /*
+     * Also zap the stub mapping for this CPU, if no other online one uses
+     * the same page.
+     */
+    if ( stub_linear )
+    {
+        unsigned int other;
+
+        for_each_online_cpu(other)
+            if ( !((per_cpu(stubs.addr, other) ^ stub_linear) >> PAGE_SHIFT) )
+            {
+                stub_linear = 0;
+                break;
+            }
+    }
     if ( stub_linear )
     {
         l3_pgentry_t *l3t = l4e_to_l3e(common_pgt);





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