powerpc/time: Fix KVM host re-arming a timer beyond decrementer range

If the next host timer is beyond decrementer range, timer_rearm_host_dec
will leave decrementer not programmed. This will not cause a problem for
the host it will just set the decrementer correctly when the decrementer
interrupt hits, it seems safer not to leave the next host decrementer
interrupt timing able to be influenced by a guest.

This code is only used in the P9 KVM paths so it's unlikely to be hit
practically unless large decrementer is force disabled in the host.

Fixes: 25aa145 ("powerpc/time: add API for KVM to re-arm the host timer/decrementer")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220124143930.3923442-2-npiggin@gmail.com

arch/powerpc/kernel/time.c

@@ -582,8 +582,9 @@ void timer_rearm_host_dec(u64 now)
 		local_paca->irq_happened |= PACA_IRQ_DEC;
 	} else {
 		now = *next_tb - now;
-		if (now <= decrementer_max)
-			set_dec_or_work(now);
+		if (now > decrementer_max)
+			now = decrementer_max;
+		set_dec_or_work(now);
 	}
 }
 EXPORT_SYMBOL_GPL(timer_rearm_host_dec);