accel/tcg: Support split-wx for linux with memfd

We cannot use a real temp file, because we would need to find
a filesystem that does not have noexec enabled.  However, a
memfd is not associated with any filesystem.

Reviewed-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 76 insertions, 8 deletions

diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index 022f9c7..3b68045 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -1078,17 +1078,11 @@ static bool alloc_code_gen_buffer(size_t size, int splitwx, Error **errp)
     return true;
 }
 #else
-static bool alloc_code_gen_buffer(size_t size, int splitwx, Error **errp)
+static bool alloc_code_gen_buffer_anon(size_t size, int prot,
+                                       int flags, Error **errp)
 {
-    int prot = PROT_WRITE | PROT_READ | PROT_EXEC;
-    int flags = MAP_PRIVATE | MAP_ANONYMOUS;
     void *buf;
 
-    if (splitwx > 0) {
-        error_setg(errp, "jit split-wx not supported");
-        return false;
-    }
-
     buf = mmap(NULL, size, prot, flags, -1, 0);
     if (buf == MAP_FAILED) {
         error_setg_errno(errp, errno,
@@ -1137,6 +1131,80 @@ static bool alloc_code_gen_buffer(size_t size, int splitwx, Error **errp)
     tcg_ctx->code_gen_buffer = buf;
     return true;
 }
+
+#ifdef CONFIG_POSIX
+#include "qemu/memfd.h"
+
+static bool alloc_code_gen_buffer_splitwx_memfd(size_t size, Error **errp)
+{
+    void *buf_rw, *buf_rx;
+    int fd = -1;
+
+    buf_rw = qemu_memfd_alloc("tcg-jit", size, 0, &fd, errp);
+    if (buf_rw == NULL) {
+        return false;
+    }
+
+    buf_rx = mmap(NULL, size, PROT_READ | PROT_EXEC, MAP_SHARED, fd, 0);
+    if (buf_rx == MAP_FAILED) {
+        error_setg_errno(errp, errno,
+                         "failed to map shared memory for execute");
+        munmap(buf_rw, size);
+        close(fd);
+        return false;
+    }
+    close(fd);
+
+    tcg_ctx->code_gen_buffer = buf_rw;
+    tcg_ctx->code_gen_buffer_size = size;
+    tcg_splitwx_diff = buf_rx - buf_rw;
+
+    /* Request large pages for the buffer and the splitwx.  */
+    qemu_madvise(buf_rw, size, QEMU_MADV_HUGEPAGE);
+    qemu_madvise(buf_rx, size, QEMU_MADV_HUGEPAGE);
+    return true;
+}
+#endif /* CONFIG_POSIX */
+
+static bool alloc_code_gen_buffer_splitwx(size_t size, Error **errp)
+{
+    if (TCG_TARGET_SUPPORT_MIRROR) {
+#ifdef CONFIG_POSIX
+        return alloc_code_gen_buffer_splitwx_memfd(size, errp);
+#endif
+    }
+    error_setg(errp, "jit split-wx not supported");
+    return false;
+}
+
+static bool alloc_code_gen_buffer(size_t size, int splitwx, Error **errp)
+{
+    ERRP_GUARD();
+    int prot, flags;
+
+    if (splitwx) {
+        if (alloc_code_gen_buffer_splitwx(size, errp)) {
+            return true;
+        }
+        /*
+         * If splitwx force-on (1), fail;
+         * if splitwx default-on (-1), fall through to splitwx off.
+         */
+        if (splitwx > 0) {
+            return false;
+        }
+        error_free_or_abort(errp);
+    }
+
+    prot = PROT_READ | PROT_WRITE | PROT_EXEC;
+    flags = MAP_PRIVATE | MAP_ANONYMOUS;
+#ifdef CONFIG_TCG_INTERPRETER
+    /* The tcg interpreter does not need execute permission. */
+    prot = PROT_READ | PROT_WRITE;
+#endif
+
+    return alloc_code_gen_buffer_anon(size, prot, flags, errp);
+}
 #endif /* USE_STATIC_CODE_GEN_BUFFER, WIN32, POSIX */
 
 static bool tb_cmp(const void *ap, const void *bp)