x86: don't use destroy_xen_mappings() for vunmap()

Its attempt to tear down intermediate page table levels may race with
map_pages_to_xen() establishing them, and now that
map_domain_page_global() is backed by vmap() this teardown is also
wasteful (as it's very likely to need the same address space populated
again within foreseeable time).

As the race between vmap() and vunmap(), according to the latest stage
tester logs, doesn't appear to be the only one still left, the patch
also adds logging for vmap() and vunmap() uses (there shouldn't be too
many of them, so logs shouldn't get flooded). These are supposed to
get removed (and are made stand out clearly) as soon as we're certain
that there's no issue left.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/vmap.c
+++ b/xen/common/vmap.c
@@ -177,6 +177,7 @@ void *__vmap(const unsigned long *mfn, u
     void *va = vm_alloc(nr * granularity, align);
     unsigned long cur = (unsigned long)va;
 
+printk("vmap(%p:%#x)\n", va, nr * granularity);//temp
     for ( ; va && nr--; ++mfn, cur += PAGE_SIZE * granularity )
     {
         if ( map_pages_to_xen(cur, *mfn, granularity, flags) )
@@ -196,9 +197,14 @@ void *vmap(const unsigned long *mfn, uns
 
 void vunmap(const void *va)
 {
+#ifndef _PAGE_NONE
     unsigned long addr = (unsigned long)va;
 
     destroy_xen_mappings(addr, addr + PAGE_SIZE * vm_size(va));
+#else /* Avoid tearing down intermediate page tables. */
+printk("vunmap(%p:%#x)\n", va, vm_size(va));//temp
+    map_pages_to_xen((unsigned long)va, 0, vm_size(va), _PAGE_NONE);
+#endif
     vm_free(va);
 }
 #endif
--- a/xen/include/asm-x86/page.h
+++ b/xen/include/asm-x86/page.h
@@ -288,6 +288,7 @@ extern l1_pgentry_t l1_identmap[L1_PAGET
 void paging_init(void);
 #endif /* !defined(__ASSEMBLY__) */
 
+#define _PAGE_NONE     _AC(0x000,U)
 #define _PAGE_PRESENT  _AC(0x001,U)
 #define _PAGE_RW       _AC(0x002,U)
 #define _PAGE_USER     _AC(0x004,U)