target/ppc: Fix decrementer time underflow and infinite timer loop

It is possible to store a very large value to the decrementer that it
does not raise the decrementer exception so the timer is scheduled, but
the next time value wraps and is treated as in the past.

This can occur if (u64)-1 is stored on a zero-triggered exception, or
(u64)-1 is stored twice on an underflow-triggered exception, for
example.

If such a value is set in DECAR, it gets stored to the decrementer by
the timer function, which then immediately causes another timer, which
hangs QEMU.

Clamp the decrementer to the implemented width, and use that as the
value for the timer calculation, effectively preventing this overflow.

Reported-by: sdicaro@DDCI.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230530131214.373524-1-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
(cherry picked from commit 09d2db9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/ppc/ppc.c

@@ -806,6 +806,8 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
     int64_t signed_decr;
 
     /* Truncate value to decr_width and sign extend for simplicity */
+    value = extract64(value, 0, nr_bits);
+    decr = extract64(decr, 0, nr_bits);
     signed_value = sextract64(value, 0, nr_bits);
     signed_decr = sextract64(decr, 0, nr_bits);