[request for review] cmpxchg8b asm stuff

[Gerd Knorr <kraxel@bytesex.org>]

  Hi,

Below is my very first attempt to write inline assembler, for atomic
updates of PAE pagetable entries using cmpxchg8b.  It builds, but is
completely untested otherwise as I don't have a PAE-enabled guest kernel
yet ...

Can someone with more experience than me please have a look at it?

Thanks,

  Gerd

--- xen.orig/arch/x86/mm.c	2005-04-19 11:56:20.000000000 +0200
+++ xen/arch/x86/mm.c	2005-04-19 15:29:01.000000000 +0200
@@ -834,12 +846,55 @@ static void free_l4_table(struct pfn_inf
 
 #endif /* __x86_64__ */
 
+static inline int cmpxchg8b_user(u64 *ptr, u64 oval, u64 nval)
+{
+    u32 o_hi = oval >> 32;
+    u32 o_lo = oval & 0xffffffff;
+    u32 n_hi = nval >> 32;
+    u32 n_lo = nval & 0xffffffff;
+    u32 rc = 0;
+
+    __asm__ __volatile__ (
+        "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"
+        "2:\n"
+        ".section .fixup,\"ax\"\n"
+        "3:     movl $1, %[rc]\n"
+        "       jmp 2b\n"
+        ".previous\n"
+        ".section __ex_table,\"a\"\n"
+        "       .align 4\n"
+        "       .long 1b,3b\n"
+        ".previous"
+        : "=d" (o_hi), "=a" (o_lo), "=c" (n_hi), "=b" (n_lo),
+          [rc] "=r" (rc), [ptr] "m" (*__xg((volatile void *)ptr))
+        : "0" (o_hi), "1" (o_lo)
+        : "memory");
+    if ((o_hi != (oval >> 32)) ||
+        (o_lo != (oval & 0xffffffff)))
+        rc = 1;
+    return rc;
+}
 
 static inline int update_l1e(l1_pgentry_t *pl1e, 
                              l1_pgentry_t  ol1e, 
                              l1_pgentry_t  nl1e)
 {
-    /* FIXME: breaks with PAE */
+#if defined(__i386__) && defined(CONFIG_X86_PAE)
+
+    int rc;
+
+    rc = cmpxchg8b_user(pl1e, l1e_get_value(ol1e),
+                        l1e_get_value(nl1e));
+    if (unlikely(rc))
+    {
+        MEM_LOG("Failed to update %llx -> %llx [pae]\n",
+                l1e_get_value(ol1e), l1e_get_value(nl1e));
+        return 0;
+    }
+    return 1;
+
+#else
+
     unsigned long o = l1e_get_value(ol1e);
     unsigned long n = l1e_get_value(nl1e);
 
@@ -850,8 +905,9 @@ static inline int update_l1e(l1_pgentry_
                 l1e_get_value(ol1e), l1e_get_value(nl1e), o);
         return 0;
     }
-
     return 1;
+
+#endif
 }