[Xen-devel] [PATCH] x86/setup: Correct register clobbers for the asm statement when resyncing the stack

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

When resyncing the stack, the asm statement does not identify %rsi, %rdi and
%rcx as clobbered by the 'rep movsq'.

Luckily, there are no functional problems in the generated code.  GCC decides
not to save any of them before calling boostrap_map(), which clobbers them.

Correct the clobbers, by listing them as earlyclobber discarded outputs.

Reported-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>

---

I have done an audit of the other uses of `rep $STRINGOP` and this is the only
asm statement with incorrect clobbers.
---
 xen/arch/x86/setup.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 8c8b91f..f470c4a 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -836,6 +836,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
             l2_pgentry_t *pl2e;
             uint64_t load_start;
             int i, j, k;
+            long _discard;
 
             /* Select relocation address. */
             e = end - reloc_size;
@@ -902,11 +903,13 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                 "movq %%cr4,%%rsi ; "
                 "andb $0x7f,%%sil ; "
                 "movq %%rsi,%%cr4 ; " /* CR4.PGE == 0 */
-                "movq %0,%%cr3    ; " /* CR3 == new pagetables */
+                "movq %6,%%cr3    ; " /* CR3 == new pagetables */
                 "orb $0x80,%%sil  ; "
                 "movq %%rsi,%%cr4   " /* CR4.PGE == 1 */
-                : : "r" (__pa(idle_pg_table)), "S" (cpu0_stack),
-                "D" (__va(__pa(cpu0_stack))), "c" (STACK_SIZE / 8) : "memory" 
);
+                : "=&S"(_discard), "=&D"(_discard), "=&c"(_discard)
+                : "0"(cpu0_stack), "1"(__va(__pa(cpu0_stack))),
+                  "2"(STACK_SIZE / 8), "r"(__pa(idle_pg_table))
+                : "memory" );
 
             bootstrap_map(NULL);
         }
-- 
1.7.10.4


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