[Xen-devel] [PATCH v2 4/5] x86/boot: Simplify pagetable manipulation loops

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

For __page_tables_{start,end} and L3 bootmap initialisation, the logic is
unnecesserily complicated owing to its attempt to use the LOOP instruction,
which results in an off-by-8 memory address owing to LOOP's termination
condition.

Rewrite both loops for improved clarity and speed.

Misc notes:
 * TEST $IMM, MEM can't macrofuse.  The loop has 0x1200 iterations, so pull
   the $_PAGE_PRESENT constant out into a spare register to turn the TEST into
   its %REG, MEM form, which can macrofuse.
 * Avoid the use of %fs-relative references.  %esi-relative is the more common
   form in the code, and doesn't suffer an address generation overhead.
 * Avoid LOOP.  CMP/JB isn't microcoded and faster to execute in all cases.
 * For a 4 interation trivial loop, even compilers unroll these.  The
   generated code size is a fraction larger, but this is init and the asm is
   far easier to follow.
 * Reposition the l2=>l1 bootmap construction so the asm reads in pagetable
   level order.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/boot/head.S | 41 ++++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 15 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 1deeae2f2a..1acaf817ba 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -662,11 +662,17 @@ trampoline_setup:
         mov     %edx,sym_fs(boot_tsc_stamp)+4
 
         /* Relocate pagetables to point at Xen's current location in memory. */
-        mov     $((__page_tables_end-__page_tables_start)/8),%ecx
-1:      testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
+        mov     $_PAGE_PRESENT, %edx
+        lea     sym_esi(__page_tables_start), %eax
+        lea     sym_esi(__page_tables_end), %edi
+
+1:      testb   %dl, (%eax)  /* if page present */
         jz      2f
-        add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
-2:      loop    1b
+        add     %esi, (%eax) /* pte += base */
+2:      add     $8, %eax
+
+        cmp     %edi, %eax
+        jb      1b
 
         /* Map Xen into the higher mappings using 2M superpages. */
         lea     _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), %eax
@@ -701,22 +707,27 @@ trampoline_setup:
         cmp     %edx, %ecx
         jbe     1b
 
-        /* Initialize L3 boot-map page directory entries. */
-        lea     
__PAGE_HYPERVISOR+(L2_PAGETABLE_ENTRIES*8)*3+sym_esi(l2_bootmap),%eax
-        mov     $4,%ecx
-1:      mov     %eax,sym_fs(l3_bootmap)-8(,%ecx,8)
-        sub     $(L2_PAGETABLE_ENTRIES*8),%eax
-        loop    1b
-
-        /* Map the permanent trampoline page into l{1,2}_bootmap[]. */
+        /* Map 4x l2_bootmap[] into l3_bootmap[0...3] */
+        lea     __PAGE_HYPERVISOR + sym_esi(l2_bootmap), %eax
+        mov     $PAGE_SIZE, %edx
+        mov     %eax, 0  + sym_esi(l3_bootmap)
+        add     %edx, %eax
+        mov     %eax, 8  + sym_esi(l3_bootmap)
+        add     %edx, %eax
+        mov     %eax, 16 + sym_esi(l3_bootmap)
+        add     %edx, %eax
+        mov     %eax, 24 + sym_esi(l3_bootmap)
+
+        /* Map l1_bootmap[] into l2_bootmap[0]. */
+        lea     __PAGE_HYPERVISOR + sym_esi(l1_bootmap), %eax
+        mov     %eax, sym_esi(l2_bootmap)
+
+        /* Map the permanent trampoline page into l1_bootmap[]. */
         mov     sym_esi(trampoline_phys), %ecx
         lea     __PAGE_HYPERVISOR_RX(%ecx), %edx /* %edx = PTE to write  */
         shr     $PAGE_SHIFT, %ecx                /* %ecx = Slot to write */
         mov     %edx, sym_offs(l1_bootmap)(%esi, %ecx, 8)
 
-        lea     __PAGE_HYPERVISOR + sym_esi(l1_bootmap), %edx
-        mov     %edx, sym_esi(l2_bootmap)
-
         /* Apply relocations to bootstrap trampoline. */
         mov     sym_esi(trampoline_phys), %edx
         lea     sym_esi(__trampoline_rel_start), %edi
-- 
2.11.0


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