Subject: Re: [PATCH] powerpc/time: Fix clockevent_decrementer initalisation for PR KVM

From: Anton Blanchard <[email-masked]>
Date: Sat, 20 Oct 2018 17:59:23 +1100

On Fri, 19 Oct 2018 15:23:19 +1100
Michael Ellerman <[email-masked]> wrote:

> In the recent commit 8b78fdb045de ("powerpc/time: Use
> clockevents_register_device(), fixing an issue with large
> decrementer") we changed the way we initialise the decrementer
> clockevent(s).
> 
> We no longer initialise the mult & shift values of
> decrementer_clockevent itself.
> 
> This has the effect of breaking PR KVM, because it uses those values
> in kvmppc_emulate_dec(). The symptom is guest kernels spin forever
> mid-way through boot.
> 
> For now fix it by assigning back to decrementer_clockevent the mult
> and shift values.

Thanks Michael, I missed that completely.

Acked-by: Anton Blanchard <[email-masked]>

Anton

> 
> Fixes: 8b78fdb045de ("powerpc/time: Use
> clockevents_register_device(), fixing an issue with large
> decrementer") Signed-off-by: Michael Ellerman <[email-masked]> ---
>  arch/powerpc/kernel/time.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 40868f3ee113..68e8f963d108 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -989,6 +989,10 @@ static void register_decrementer_clockevent(int
> cpu) 
>  	printk_once(KERN_DEBUG "clockevent: %s mult[%x] shift[%d]
> cpu[%d]\n", dec->name, dec->mult, dec->shift, cpu);
> +
> +	/* Set values for KVM, see kvm_emulate_dec() */
> +	decrementer_clockevent.mult = dec->mult;
> +	decrementer_clockevent.shift = dec->shift;
>  }
>  
>  static void enable_large_decrementer(void)