accel/tcg: avoid re-translating one-shot instructions

By definition a single instruction is capable of being an IO
instruction. This avoids a problem of triggering a cpu_io_recompile on
a non-recorded translation which then fails because it expects
tcg_tb_lookup() to succeed unconditionally. The normal use case
requires a TB to be able to resolve machine state.

The other users of tcg_tb_lookup() are able to tolerate a missing TB
if the machine state has been resolved by other means - which in the
single-shot case is always true because machine state is synced at the
start of a block.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210415162454.22056-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

accel/tcg/translate-all.c

@@ -1863,7 +1863,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
 
     if (phys_pc == -1) {
         /* Generate a one-shot TB with 1 insn in it */
-        cflags = (cflags & ~CF_COUNT_MASK) | 1;
+        cflags = (cflags & ~CF_COUNT_MASK) | CF_LAST_IO | 1;
     }
 
     max_insns = cflags & CF_COUNT_MASK;