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: 25aa145856cd ("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

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 958e2929776f4160ec6f7afeb1ec5c7cfdfb0d57..0bb80f0bd0a6b58df6bdf3cf01489464ec2187fe 100644 (file)
--- a/arch/powerpc/kernel/time.c
+++ b/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);