[PATCH 3/3] x86/boot: Don't double-copy the stack during physical relocation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

cpu0_stack is contained within .data, which means the memcpy() already takes a
snapshot at the start of the critical region.

Later, when we switch to the relocated Xen, we do end up losing updates to the
local variables, but that's fine because the only variables we've modified go
out of scope after the printk().  Use this properly to avoid copying the whole
stack (32k) a second time.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

Ever so slightly RFC, as it has only had light testing, but I'm confident in
the reasoning.
---
 xen/arch/x86/setup.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index a6ff450daab7..c04c68a09b47 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1183,6 +1183,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
             l3_pgentry_t *pl3e;
             l2_pgentry_t *pl2e;
             int i, j, k;
+            unsigned long tmp;
 
             /* Select relocation address. */
             xen_phys_start = end - reloc_size;
@@ -1193,7 +1194,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
              * Perform relocation to new physical address.
              * Before doing so we must sync static/global data with main memory
              * with a barrier(). After this we must *not* modify static/global
-             * data until after we have switched to the relocated pagetables!
+             * data, or locals that need to survive, until after we have
+             * switched to the relocated pagetables!
              */
             barrier();
             memcpy(__va(__pa(_start)), _start, _end - _start);
@@ -1239,18 +1241,21 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                 *pl2e = l2e_from_intpte(l2e_get_intpte(*pl2e) + 
xen_phys_start);
             }
 
-            /* Re-sync the stack and then switch to relocated pagetables. */
+            /*
+             * Switch to relocated pagetables.  This also discards updates to
+             * any local variables since the memmove() call above, but that's
+             * fine because we don't use any of them again.
+             */
             asm volatile (
-                "rep movsq        ; " /* re-sync the stack */
-                "movq %%cr4,%%rsi ; "
-                "andb $0x7f,%%sil ; "
-                "movq %%rsi,%%cr4 ; " /* CR4.PGE == 0 */
-                "movq %[pg],%%cr3 ; " /* CR3 == new pagetables */
-                "orb $0x80,%%sil  ; "
-                "movq %%rsi,%%cr4   " /* CR4.PGE == 1 */
-                : "=&S" (i), "=&D" (i), "=&c" (i) /* All outputs discarded. */
-                :  [pg] "r" (__pa(idle_pg_table)), "0" (cpu0_stack),
-                   "1" (__va(__pa(cpu0_stack))), "2" (STACK_SIZE / 8)
+                "mov    %%cr4, %[cr4]\n\t"
+                "andb   $~%c[pge], %b[cr4]\n\t"
+                "mov    %[cr4], %%cr4\n\t"     /* CR4.PGE = 0 */
+                "mov    %[cr3], %%cr3\n\t"     /* CR3 = new pagetables */
+                "orb    $%c[pge], %b[cr4]\n\t"
+                "mov    %[cr4], %%cr4\n\t"     /* CR4.PGE = 1 */
+                : [cr4] "=&a" (tmp) /* Could be "r", but "a" makes better asm 
*/
+                : [cr3] "r" (__pa(idle_pg_table)),
+                  [pge] "i" (X86_CR4_PGE)
                 : "memory" );
 
             printk("New Xen image base address: %#lx\n", xen_phys_start);
-- 
2.11.0