[PATCH 3/3] linux-user/arm: Implement __kernel_cmpxchg64 with host atomics

[Richard Henderson <richard.henderson@linaro.org>]

If CONFIG_ATOMIC64, we can use a host cmpxchg and provide
atomicity across processes; otherwise we have no choice but
to continue using start/end_exclusive.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 linux-user/arm/cpu_loop.c | 79 +++++++++++++++++++--------------------
 1 file changed, 38 insertions(+), 41 deletions(-)

diff --git a/linux-user/arm/cpu_loop.c b/linux-user/arm/cpu_loop.c
index 0122bb34f7..d9651f199f 100644
--- a/linux-user/arm/cpu_loop.c
+++ b/linux-user/arm/cpu_loop.c
@@ -136,7 +136,7 @@ static void arm_kernel_cmpxchg32_helper(CPUARMState *env)
 }
 
 /*
- * See the Linux kernel's Documentation/arm/kernel_user_helpers.txt
+ * See the Linux kernel's Documentation/arm/kernel_user_helpers.rst
  * Input:
  * r0 = pointer to oldval
  * r1 = pointer to newval
@@ -153,57 +153,54 @@ static void arm_kernel_cmpxchg64_helper(CPUARMState *env)
 {
     uint64_t oldval, newval, val;
     uint32_t addr, cpsr;
+    uint64_t *host_addr;
 
-    /* Based on the 32 bit code in do_kernel_trap */
+    addr = env->regs[0];
+    if (get_user_u64(oldval, addr)) {
+        goto segv;
+    }
 
-    /* XXX: This only works between threads, not between processes.
-       It's probably possible to implement this with native host
-       operations. However things like ldrex/strex are much harder so
-       there's not much point trying.  */
-    start_exclusive();
-    cpsr = cpsr_read(env);
+    addr = env->regs[1];
+    if (get_user_u64(newval, addr)) {
+        goto segv;
+    }
+
+    mmap_lock();
     addr = env->regs[2];
-
-    if (get_user_u64(oldval, env->regs[0])) {
-        env->exception.vaddress = env->regs[0];
-        goto segv;
-    };
-
-    if (get_user_u64(newval, env->regs[1])) {
-        env->exception.vaddress = env->regs[1];
-        goto segv;
-    };
-
-    if (get_user_u64(val, addr)) {
-        env->exception.vaddress = addr;
-        goto segv;
+    host_addr = atomic_mmu_lookup(env, addr, 8);
+    if (!host_addr) {
+        mmap_unlock();
+        return;
     }
 
+#ifdef CONFIG_ATOMIC64
+    val = qatomic_cmpxchg__nocheck(host_addr, oldval, newval);
+    cpsr = (val == oldval) * CPSR_C;
+#else
+    /*
+     * This only works between threads, not between processes, but since
+     * the host has no 64-bit cmpxchg, it is the best that we can do.
+     */
+    start_exclusive();
+    val = *host_addr;
     if (val == oldval) {
-        val = newval;
-
-        if (put_user_u64(val, addr)) {
-            env->exception.vaddress = addr;
-            goto segv;
-        };
-
-        env->regs[0] = 0;
-        cpsr |= CPSR_C;
+        *host_addr = newval;
+        cpsr = CPSR_C;
     } else {
-        env->regs[0] = -1;
-        cpsr &= ~CPSR_C;
+        cpsr = 0;
     }
-    cpsr_write(env, cpsr, CPSR_C, CPSRWriteByInstr);
     end_exclusive();
+#endif
+    mmap_unlock();
+
+    cpsr_write(env, cpsr, CPSR_C, CPSRWriteByInstr);
+    env->regs[0] = cpsr ? 0 : -1;
     return;
 
-segv:
-    end_exclusive();
-    /* We get the PC of the entry address - which is as good as anything,
-       on a real kernel what you get depends on which mode it uses. */
-    /* XXX: check env->error_code */
-    force_sig_fault(TARGET_SIGSEGV, TARGET_SEGV_MAPERR,
-                    env->exception.vaddress);
+ segv:
+    force_sig_fault(TARGET_SIGSEGV,
+                    page_get_flags(addr) & PAGE_VALID ?
+                    TARGET_SEGV_ACCERR : TARGET_SEGV_MAPERR, addr);
 }
 
 /* Handle a jump to the kernel code page.  */
-- 
2.25.1