[RFC PATCH 09/11] powerpc/tm: Do not restore default DSCR

[Michael Neuling]

On Wed, 2018-09-12 at 16:40 -0300, Breno Leitao wrote:
> In the previous TM code, trecheckpoint was being executed in the middle of
> an exception, thus, DSCR was being restored to default kernel DSCR value
> after trecheckpoint was done.
> 
> With this current patchset, trecheckpoint is executed just before getting
> to userspace, at ret_from_except_lite, for example. Thus, we do not need to
> set default kernel DSCR value anymore, as we are leaving kernel space.  It
> is OK to keep the checkpointed DSCR value into the live SPR, mainly because
> the transaction is doomed and it will fail soon (after RFID), 

What if we are going back to a suspended transaction?  It will remain live until
userspace does a tresume

> so,
> continuing with the pre-checkpointed DSCR value is what seems correct.

Reading this description suggests this patch isn't really needed. Right?

Mikey

> That said, we must set the DSCR value that will be used in userspace now.
> Current trecheckpoint() function sets it to the pre-checkpointed value
> prior to lines being changed in this patch, so, removing these lines would
> keep the pre-checkpointed values.
> 
> Important to say that we do not need to do the same thing with tm_reclaim,
> since it already set the DSCR to the default value, after TRECLAIM is
> called, in the following lines:
> 
>         /* Load CPU's default DSCR */
>         ld      r0, PACA_DSCR_DEFAULT(r13)
>         mtspr   SPRN_DSCR, r0
> 
> Signed-off-by: Breno Leitao <leitao at debian.org>
> ---
>  arch/powerpc/kernel/tm.S | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/tm.S b/arch/powerpc/kernel/tm.S
> index 6bffbc5affe7..5427eda69846 100644
> --- a/arch/powerpc/kernel/tm.S
> +++ b/arch/powerpc/kernel/tm.S
> @@ -493,10 +493,6 @@ restore_gprs:
>  	mtlr	r0
>  	ld	r2, STK_GOT(r1)
>  
> -	/* Load CPU's default DSCR */
> -	ld	r0, PACA_DSCR_DEFAULT(r13)
> -	mtspr	SPRN_DSCR, r0
> -
>  	blr
>  
>  	/* ****************************************************************** */