linux-user: Set CF_PARALLEL when mapping shared memory

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>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210612060828.695332-1-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

1 files changed, 14 insertions, 0 deletions

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 7e3b245..0e10385 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;