accel/tcg: mttcg remove false-negative halted assertion

mttcg asserts that an execution ending with EXCP_HALTED must have
cpu->halted. However between the event or instruction that sets
cpu->halted and requests exit and the assertion here, an
asynchronous event could clear cpu->halted.

This leads to crashes running AIX on ppc/pseries because it uses
H_CEDE/H_PROD hcalls, where H_CEDE sets self->halted = 1 and
H_PROD sets other cpu->halted = 0 and kicks it.

H_PROD could be turned into an interrupt to wake, but several other
places in ppc, sparc, and semihosting follow what looks like a similar
pattern setting halted = 0 directly. So remove this assertion.

Reported-by: Ivan Warren <ivan@vmfacility.fr>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20230829010658.8252-1-npiggin@gmail.com>
[rth: Keep the case label and adjust the comment.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 0e5903436d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

accel/tcg/tcg-accel-ops-mttcg.c

@@ -100,14 +100,9 @@ static void *mttcg_cpu_thread_fn(void *arg)
                 break;
             case EXCP_HALTED:
                 /*
-                 * during start-up the vCPU is reset and the thread is
-                 * kicked several times. If we don't ensure we go back
-                 * to sleep in the halted state we won't cleanly
-                 * start-up when the vCPU is enabled.
-                 *
-                 * cpu->halted should ensure we sleep in wait_io_event
+                 * Usually cpu->halted is set, but may have already been
+                 * reset by another thread by the time we arrive here.
                  */
-                g_assert(cpu->halted);
                 break;
             case EXCP_ATOMIC:
                 qemu_mutex_unlock_iothread();