[PATCH 1/2] x86: drop unnecessary page table walking in compat r/o M2P handling

[Jan Beulich <jbeulich@xxxxxxxx>]

We have a global variable where the necessary L2 table is recorded; no
need to inspect L4 and L3 tables (and this way a few less places will
eventually need adjustment when we want to support 5-level page tables).
Also avoid setting up the L3 entry, as the address range never gets used
anyway (it'll be dropped altogether in a subsequent patch).

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

--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -219,9 +219,7 @@ static void destroy_compat_m2p_mapping(s
 {
     unsigned long i, va, rwva, pt_pfn;
     unsigned long smap = info->spfn, emap = info->spfn;
-
-    l3_pgentry_t *l3_ro_mpt;
-    l2_pgentry_t *l2_ro_mpt;
+    l2_pgentry_t *l2_ro_mpt = compat_idle_pg_table_l2;
 
     if ( smap > ((RDWR_COMPAT_MPT_VIRT_END - RDWR_COMPAT_MPT_VIRT_START) >> 2) 
)
         return;
@@ -229,12 +227,6 @@ static void destroy_compat_m2p_mapping(s
     if ( emap > ((RDWR_COMPAT_MPT_VIRT_END - RDWR_COMPAT_MPT_VIRT_START) >> 2) 
)
         emap = (RDWR_COMPAT_MPT_VIRT_END - RDWR_COMPAT_MPT_VIRT_START) >> 2;
 
-    l3_ro_mpt = 
l4e_to_l3e(idle_pg_table[l4_table_offset(HIRO_COMPAT_MPT_VIRT_START)]);
-
-    
ASSERT(l3e_get_flags(l3_ro_mpt[l3_table_offset(HIRO_COMPAT_MPT_VIRT_START)]) & 
_PAGE_PRESENT);
-
-    l2_ro_mpt = 
l3e_to_l2e(l3_ro_mpt[l3_table_offset(HIRO_COMPAT_MPT_VIRT_START)]);
-
     for ( i = smap; i < emap; )
     {
         va = HIRO_COMPAT_MPT_VIRT_START +
@@ -323,7 +315,6 @@ static int setup_compat_m2p_table(struct
     unsigned long i, va, smap, emap, rwva, epfn = info->epfn;
     mfn_t mfn;
     unsigned int n;
-    l3_pgentry_t *l3_ro_mpt = NULL;
     l2_pgentry_t *l2_ro_mpt = NULL;
     int err = 0;
 
@@ -342,13 +333,7 @@ static int setup_compat_m2p_table(struct
     emap = ( (epfn + ((1UL << (L2_PAGETABLE_SHIFT - 2)) - 1 )) &
                 ~((1UL << (L2_PAGETABLE_SHIFT - 2)) - 1) );
 
-    va = HIRO_COMPAT_MPT_VIRT_START +
-         smap * sizeof(*compat_machine_to_phys_mapping);
-    l3_ro_mpt = l4e_to_l3e(idle_pg_table[l4_table_offset(va)]);
-
-    ASSERT(l3e_get_flags(l3_ro_mpt[l3_table_offset(va)]) & _PAGE_PRESENT);
-
-    l2_ro_mpt = l3e_to_l2e(l3_ro_mpt[l3_table_offset(va)]);
+    l2_ro_mpt = compat_idle_pg_table_l2;
 
 #define MFN(x) (((x) << L2_PAGETABLE_SHIFT) / sizeof(unsigned int))
 #define CNT ((sizeof(*frame_table) & -sizeof(*frame_table)) / \
@@ -627,16 +612,10 @@ void __init paging_init(void)
 #undef MFN
 
     /* Create user-accessible L2 directory to map the MPT for compat guests. */
-    BUILD_BUG_ON(l4_table_offset(RDWR_MPT_VIRT_START) !=
-                 l4_table_offset(HIRO_COMPAT_MPT_VIRT_START));
-    l3_ro_mpt = l4e_to_l3e(idle_pg_table[l4_table_offset(
-        HIRO_COMPAT_MPT_VIRT_START)]);
     if ( (l2_ro_mpt = alloc_xen_pagetable()) == NULL )
         goto nomem;
     compat_idle_pg_table_l2 = l2_ro_mpt;
     clear_page(l2_ro_mpt);
-    l3e_write(&l3_ro_mpt[l3_table_offset(HIRO_COMPAT_MPT_VIRT_START)],
-              l3e_from_paddr(__pa(l2_ro_mpt), __PAGE_HYPERVISOR_RO));
     l2_ro_mpt += l2_table_offset(HIRO_COMPAT_MPT_VIRT_START);
     /* Allocate and map the compatibility mode machine-to-phys table. */
     mpt_size = (mpt_size >> 1) + (1UL << (L2_PAGETABLE_SHIFT - 1));