[PATCH] linux-user: Set CF_PARALLEL when mapping shared memory

[Richard Henderson]

Signal the translator to use host atomic instructions for
guest operations, insofar as it is possible.  This is the
best we can do to allow the guest to interact atomically
with other processes.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/121
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 linux-user/mmap.c    | 14 ++++++++++++++
 linux-user/syscall.c | 12 ++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 7e3b245036..0e103859fe 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -451,6 +451,20 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int 
target_prot,
         goto fail;
     }
 
+    /*
+     * If we're mapping shared memory, ensure we generate code for parallel
+     * execution and flush old translations.  This will work up to the level
+     * supported by the host -- anything that requires EXCP_ATOMIC will not
+     * be atomic with respect to an external process.
+     */
+    if (flags & MAP_SHARED) {
+        CPUState *cpu = thread_cpu;
+        if (!(cpu->tcg_cflags & CF_PARALLEL)) {
+            cpu->tcg_cflags |= CF_PARALLEL;
+            tb_flush(cpu);
+        }
+    }
+
     real_start = start & qemu_host_page_mask;
     host_offset = offset & qemu_host_page_mask;
 
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 974dd46c9a..54037db8d6 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -4603,6 +4603,7 @@ static inline abi_ulong target_shmlba(CPUArchState 
*cpu_env)
 static inline abi_ulong do_shmat(CPUArchState *cpu_env,
                                  int shmid, abi_ulong shmaddr, int shmflg)
 {
+    CPUState *cpu = env_cpu(cpu_env);
     abi_long raddr;
     void *host_raddr;
     struct shmid_ds shm_info;
@@ -4633,6 +4634,17 @@ static inline abi_ulong do_shmat(CPUArchState *cpu_env,
 
     mmap_lock();
 
+    /*
+     * We're mapping shared memory, so ensure we generate code for parallel
+     * execution and flush old translations.  This will work up to the level
+     * supported by the host -- anything that requires EXCP_ATOMIC will not
+     * be atomic with respect to an external process.
+     */
+    if (!(cpu->tcg_cflags & CF_PARALLEL)) {
+        cpu->tcg_cflags |= CF_PARALLEL;
+        tb_flush(cpu);
+    }
+
     if (shmaddr)
         host_raddr = shmat(shmid, (void *)g2h_untagged(shmaddr), shmflg);
     else {
-- 
2.25.1