[RFC PATCH 08/14] powerpc/tm: Recheckpoint at exit path

[Michael Neuling]

On Tue, 2018-11-06 at 10:40 -0200, Breno Leitao wrote:
> In the past, TIF_RESTORE_TM was being handled with the rest of the TIF
> workers,
> but, that was too early, and can cause some IRQ to be replayed in suspended
> state (after recheckpoint).
> 
> This patch moves TIF_RESTORE_TM handler to as late as possible, it also
> forces the IRQ to be disabled, and it will continue to be until RFID, so,
> no IRQ will be replayed at all. I.e, if trecheckpoint happens, it will RFID
> to userspace.
> 
> This makes TIF_RESTORE_TM a special case that should not be handled
> similarly to the _TIF_USER_WORK_MASK.
> 
> Since _TIF_RESTORE_TM is not part of _TIF_USER_WORK_MASK anymore, we
> need to force system_call_exit to continue to leaves through
> fast_exception_return, so, we add the flags together with
> _TIF_USER_WORK_MASK at system_call_exist path.
> 
> If this flag is set at system_call_exit, it means that recheckpoint
> will be called, and doing it through fast_exception_return is the only
> way to do so.
> 
> Signed-off-by: Breno Leitao <leitao at debian.org>
> ---
>  arch/powerpc/include/asm/thread_info.h |  2 +-
>  arch/powerpc/kernel/entry_64.S         | 23 ++++++++++++++---------
>  2 files changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/thread_info.h
> b/arch/powerpc/include/asm/thread_info.h
> index 544cac0474cb..2835d60bc9ef 100644
> --- a/arch/powerpc/include/asm/thread_info.h
> +++ b/arch/powerpc/include/asm/thread_info.h
> @@ -139,7 +139,7 @@ void arch_setup_new_exec(void);
>  
>  #define _TIF_USER_WORK_MASK	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
>  				 _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
> -				 _TIF_RESTORE_TM | _TIF_PATCH_PENDING | \
> +				 _TIF_PATCH_PENDING | \
>  				 _TIF_FSCHECK)
>  #define _TIF_PERSYSCALL_MASK	(_TIF_RESTOREALL|_TIF_NOERROR)
>  
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 17484ebda66c..a86619edf29d 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -255,7 +255,7 @@ system_call_exit:
>  
>  	ld	r9,TI_FLAGS(r12)
>  	li	r11,-MAX_ERRNO
> -	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MAS
> K|_TIF_PERSYSCALL_MASK)
> +	andi.   r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|
> _TIF_PERSYSCALL_MASK |_TIF_RESTORE_TM)
>  	bne-	.Lsyscall_exit_work
>  
>  	andi.	r0,r8,MSR_FP
> @@ -784,14 +784,6 @@ _GLOBAL(ret_from_except_lite)
>  	SCHEDULE_USER
>  	b	ret_from_except_lite
>  2:
> -#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> -	andi.	r0,r4,_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM
> -	bne	3f		/* only restore TM if nothing else to do */
> -	addi	r3,r1,STACK_FRAME_OVERHEAD
> -	bl	restore_tm_state
> -	b	restore
> -3:
> -#endif
>  	bl	save_nvgprs
>  	/*
>  	 * Use a non volatile GPR to save and restore our thread_info flags
> @@ -938,6 +930,19 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  	 */
>  	.globl	fast_exception_return
>  fast_exception_return:
> +
> +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> +	CURRENT_THREAD_INFO(r4, r1)
> +	ld	r4,TI_FLAGS(r4)
> +	andi.   r0,r4,_TIF_RESTORE_TM
> +	beq	22f
> +	ld	r4,_MSR(r1) 	/* TODO: MSR[!PR] shouldn't be here */
> +	andi.	r0,r4,MSR_PR
> +	beq	22f  /* Skip if Kernel thread */
> +	addi	r3,r1,STACK_FRAME_OVERHEAD
> +	bl	restore_tm_state

Calling out to C here is a bit concerning this late.

The main thing you are calling out to is asm anyway (with a small C wrapper). 
We might want to adapt the asm this case, rather than the old model of getting
it called from __switch_to(). We shouldn't need to call
tm_reclaim/tm_recheckpoint from C anymore (especially if we use BUG_ON() rather
than WARN_ON() in the cases already mentioned.).

Same for patch 1.

> +22:
> +#endif
>  	ld	r3,_MSR(r1)
>  	ld	r4,_CTR(r1)
>  	ld	r0,_LINK(r1)