[PATCH-for-8.1] linux-user/arm: Do not allocate a commpage at all for M-profile CPUs

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Since commit fbd3c4cff6 ("linux-user/arm: Mark the commpage
executable") executing bare-metal (linked with rdimon.specs)
cortex-M code fails as:

  $ qemu-arm -cpu cortex-m3 ~/hello.exe.m3
  qemu-arm: ../../accel/tcg/user-exec.c:492: page_set_flags: Assertion `last <= GUEST_ADDR_MAX' failed.
  Aborted (core dumped)

Commit 4f5c67f8df ("linux-user/arm: Take more care allocating
commpage") already took care of not allocating a commpage for
M-profile CPUs, however it had to be reverted as commit 6cda41daa2.

Re-introduce the M-profile fix from commit 4f5c67f8df.

Fixes: fbd3c4cff6 ("linux-user/arm: Mark the commpage executable")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1755
Reported-by: Christophe Lyon <christophe.lyon@linaro.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 linux-user/elfload.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index d3d1352c4e..a26200d9f3 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -424,10 +424,23 @@ enum {
 
 static bool init_guest_commpage(void)
 {
-    abi_ptr commpage = HI_COMMPAGE & -qemu_host_page_size;
-    void *want = g2h_untagged(commpage);
-    void *addr = mmap(want, qemu_host_page_size, PROT_READ | PROT_WRITE,
-                      MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
+    ARMCPU *cpu = ARM_CPU(thread_cpu);
+    abi_ptr commpage;
+    void *want;
+    void *addr;
+
+    /*
+     * M-profile allocates maximum of 2GB address space, so can never
+     * allocate the commpage.  Skip it.
+     */
+    if (arm_feature(&cpu->env, ARM_FEATURE_M)) {
+        return true;
+    }
+
+    commpage = HI_COMMPAGE & -qemu_host_page_size;
+    want = g2h_untagged(commpage);
+    addr = mmap(want, qemu_host_page_size, PROT_READ | PROT_WRITE,
+                MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
 
     if (addr == MAP_FAILED) {
         perror("Allocating guest commpage");
-- 
2.38.1