Re: [PATCH RFC 2/2] ARM: nommu: remap exception base address to RAM

[Russell King - ARM Linux]

On Sun, Dec 11, 2016 at 06:42:55PM +0530, Afzal Mohammed wrote:
> Remap exception base address to start of RAM in Kernel in !MMU mode.
> 
> Based on existing Kconfig help, Kernel was expecting it to be
> configured by external support. Also earlier it was not possible to
> copy the exception table to start of RAM due to Kconfig dependency,
> which has been fixed by a change prior to this.
> 
> Kernel text start at an offset of at least 32K to account for page
> tables in MMU case. On a !MMU build too this space is kept aside, and
> since 2 pages (8K) is the maximum for exception plus stubs, it can be
> placed at the start of RAM.
> 
> Signed-off-by: Afzal Mohammed <afzal.mohd.ma@xxxxxxxxx>
> ---
> 
> i am a bit shaky about this change, though it works here on Cortex-A9,
> this probably would have to be made robust so as to not cause issue on
> other v7-A's upon trying to do !MMU (this won't affect normal MMU boot),
> or specifically where security extensions are not enabled. Also effect
> of hypervisor extension also need to be considered. Please let know if
> any better ways to handle this.
> 
> 
>  arch/arm/Kconfig-nommu       | 6 +++---
>  arch/arm/kernel/head-nommu.S | 6 ++++++
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/Kconfig-nommu b/arch/arm/Kconfig-nommu
> index b7576349528c..f57fbe3d5eb0 100644
> --- a/arch/arm/Kconfig-nommu
> +++ b/arch/arm/Kconfig-nommu
> @@ -46,9 +46,9 @@ config REMAP_VECTORS_TO_RAM
>  	  If your CPU provides a remap facility which allows the exception
>  	  vectors to be mapped to writable memory, say 'n' here.
>  
> -	  Otherwise, say 'y' here.  In this case, the kernel will require
> -	  external support to redirect the hardware exception vectors to
> -	  the writable versions located at DRAM_BASE.
> +	  Otherwise, say 'y' here.  In this case, the kernel will
> +	  redirect the hardware exception vectors to the writable
> +	  versions located at DRAM_BASE.
>  
>  config ARM_MPU
>         bool 'Use the ARM v7 PMSA Compliant MPU'
> diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> index 6b4eb27b8758..ac31c9647830 100644
> --- a/arch/arm/kernel/head-nommu.S
> +++ b/arch/arm/kernel/head-nommu.S
> @@ -158,6 +158,12 @@ __after_proc_init:
>  	bic	r0, r0, #CR_V
>  #endif
>  	mcr	p15, 0, r0, c1, c0, 0		@ write control reg
> +
> +#ifdef CONFIG_REMAP_VECTORS_TO_RAM
> +	mov	r3, #CONFIG_VECTORS_BASE	@ read VECTORS_BASE
> +	mcr	p15, 0, r3, c12, c0, 0		@ write to VBAR
> +#endif
> +

Is there really any need to do this in head.S ?  I believe it's
entirely possible to do it later - arch/arm/mm/nommu.c:paging_init().

Also, if the region setup for the vectors was moved as well, it would
then be possible to check the ID registers to determine whether this
is supported, and make the decision where to locate the vectors base
more dynamically.

That leaves one pr_notice() call using the CONFIG_VECTORS_BASE
constant...

-- 
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