[PATCH 2/2 v2] powerpc: restore dbcr0 on user space exit

[Scott Wood]

On 05/16/2013 12:34:32 AM, Bharat Bhushan wrote:
> On BookE (Branch taken + Single Step) is as same as Branch Taken
> on BookS and in Linux we simulate BookS behavior for BookE as well.
> When doing so, in Branch taken handling we want to set DBCR0_IC but
> we update the current->thread->dbcr0 and not DBCR0.
> 
> Now on 64bit the current->thread.dbcr0 (and other debug registers)
> is synchronized ONLY on context switch flow. But after handling
> Branch taken in debug exception if we return back to user space
> without context switch then single stepping change (DBCR0_ICMP)
> does not get written in h/w DBCR0 and Instruction Complete exception
> does not happen.
> 
> This fixes using ptrace reliably on BookE-PowerPC
> 
> Signed-off-by: Bharat Bhushan <bharat.bhushan at freescale.com>
> ---
> v1->v2
>  - Subject line was not having 2/2
> 
>  arch/powerpc/kernel/asm-offsets.c |    1 +
>  arch/powerpc/kernel/entry_64.S    |   24 ++++++++++++++++++++----
>  2 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/asm-offsets.c  
> b/arch/powerpc/kernel/asm-offsets.c
> index b51a97c..1e2f450 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -103,6 +103,7 @@ int main(void)
>  #endif /* CONFIG_VSX */
>  #ifdef CONFIG_PPC64
>  	DEFINE(KSP_VSID, offsetof(struct thread_struct, ksp_vsid));
> +	DEFINE(THREAD_DBCR0, offsetof(struct thread_struct, dbcr0));
>  #else /* CONFIG_PPC64 */
>  	DEFINE(PGDIR, offsetof(struct thread_struct, pgdir));
>  #if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)
> diff --git a/arch/powerpc/kernel/entry_64.S  
> b/arch/powerpc/kernel/entry_64.S
> index 794889b..561630d 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -614,7 +614,9 @@ _GLOBAL(ret_from_except_lite)
>  	 * from the interrupt.
>  	 */
>  #ifdef CONFIG_PPC_BOOK3E
> +	ld	r3,PACACURRENT(r13)
>  	wrteei	0
> +	lwz	r10,(THREAD+THREAD_DBCR0)(r3)

I know I asked you to move these earlier, but this is probably too  
early... wrteei has synchronization, so it will probably have to wait  
until the ld completes, defeating the purpose of moving it earlier.

Ideal would probably be to load PACACURRENT immediately after _MSR(r1),  
and load DBCR0 just after "beq resume_kernel".

Or, move DBCR0 to therad_info as I suggested internally.

Regardless of what you do, could you run a basic syscall benchmark  
(e.g. from lmbench) before and after the patch?

-Scott