Subject: Re: [PATCH] Make system_reset_pSeries relocatable

From: Nicholas Piggin <[email-masked]>
Date: Wed, 27 Jul 2016 21:50:03 +1000

On Wed, 27 Jul 2016 17:32:06 +1000
Balbir Singh <[email-masked]> wrote:

> From: Balbir Singh <[email-masked]>
> 
> Currently the power management bits are broken w.r.t. relocation.
> There are direct branches from system_reset_pSeries to
> power7_wakeup_*. The correct way to do it is to do what
> the slb miss handler does, which is jump to a small stub within
> the first 64k of the relocated address and then jump to the
> actual location.
> 
> The code has been lightly tested (not the kvm bits), I would highly
> appreciate a review of the code. I suspect there might be easy
> to find bugs :)
> 
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> 
> Signed-off-by: Balbir Singh <[email-masked]>
> ---
>  arch/powerpc/kernel/exceptions-64s.S | 82
> ++++++++++++++++++++++-------------- 1 file changed, 51
> insertions(+), 31 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/exceptions-64s.S
> b/arch/powerpc/kernel/exceptions-64s.S index 8bcc1b4..64f9650 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -118,39 +118,21 @@ BEGIN_FTR_SECTION
>  	cmpwi	cr4,r5,1
>  	mtspr	SPRN_HSPRG0,r13
>  
> -	lbz	r0,PACA_THREAD_IDLE_STATE(r13)
> -	cmpwi   cr2,r0,PNV_THREAD_NAP
> -	bgt     cr2,8f				/* Either
> sleep or Winkle */ -
> -	/* Waking up from nap should not cause hypervisor state loss
> */
> -	bgt	cr3,.
> -
> -	/* Waking up from nap */
> -	li	r0,PNV_THREAD_RUNNING
> -	stb	r0,PACA_THREAD_IDLE_STATE(r13)	/* Clear
> thread state */ -
> -#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
> -	li	r0,KVM_HWTHREAD_IN_KERNEL
> -	stb	r0,HSTATE_HWTHREAD_STATE(r13)
> -	/* Order setting hwthread_state vs. testing hwthread_req */
> -	sync
> -	lbz	r0,HSTATE_HWTHREAD_REQ(r13)
> -	cmpwi	r0,0
> -	beq	1f
> -	b	kvm_start_guest
> -1:
> +#ifndef CONFIG_RELOCATABLE
> +	b	power7_wakeup_common
> +#else
> +	/*
> +	 * We can't just use a direct branch to power7_wakeup_common
> +	 * because the distance from here to there depends on where
> +	 * the kernel ends up being put.
> +	 */
> +	mfctr	r11
> +	ld	r10, PACAKBASE(r13)
> +	LOAD_HANDLER(r10, power7_wakeup_common)
> +	mtctr	r10
> +	bctr
>  #endif

So r10 and r11 are safe to use (as well as existing registers
being used without saving) because we are returning via the nap
functions that caller will expect to trash volatile registers,
yes?

In that case I can't see a problem with this.

Thanks,
Nick