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Date:   Thu, 06 Jun 2019 13:51:20 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from
 exception
To:     mpe@ellerman.id.au, Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        mahesh@linux.vnet.ibm.com, mikey@neuling.org, npiggin@gmail.com,
        paulus@samba.org
References: <20190606072951.32116-1-ravi.bangoria@linux.ibm.com>
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Ravi Bangoria wrote:
> Powerpc hw triggers watchpoint before executing the instruction.
> To make trigger-after-execute behavior, kernel emulates the
> instruction. If the instruction is 'load something into non-
> volatile register', exception handler should restore emulated
> register state while returning back, otherwise there will be
> register state corruption. Ex, Adding a watchpoint on a list
> can corrput the list:
> 
>   # cat /proc/kallsyms | grep kthread_create_list
>   c00000000121c8b8 d kthread_create_list
> 
> Add watchpoint on kthread_create_list->next:
> 
>   # perf record -e mem:0xc00000000121c8c0
> 
> Run some workload such that new kthread gets invoked. Ex, I
> just logged out from console:
> 
>   list_add corruption. next->prev should be prev (c000000001214e00), \
> 	but was c00000000121c8b8. (next=c00000000121c8b8).
>   WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
>   CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
>   ...
>   NIP __list_add_valid+0xb4/0xc0
>   LR __list_add_valid+0xb0/0xc0
>   Call Trace:
>   __list_add_valid+0xb0/0xc0 (unreliable)
>   __kthread_create_on_node+0xe0/0x260
>   kthread_create_on_node+0x34/0x50
>   create_worker+0xe8/0x260
>   worker_thread+0x444/0x560
>   kthread+0x160/0x1a0
>   ret_from_kernel_thread+0x5c/0x70
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
>  arch/powerpc/kernel/exceptions-64s.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Awesome catch - this one has had a glorious run...
Fixes: 5aae8a5370802 ("powerpc, hw_breakpoints: Implement hw_breakpoints for 64-bit server processors")

Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>


- Naveen