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Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from
 exception
To:     Michael Neuling <mikey@neuling.org>, mpe@ellerman.id.au
Cc:     benh@kernel.crashing.org, paulus@samba.org, npiggin@gmail.com,
        christophe.leroy@c-s.fr, mahesh@linux.vnet.ibm.com,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
References: <20190606072951.32116-1-ravi.bangoria@linux.ibm.com>
 <80cfc8d7327d3bb744ea1f7e2843943a998d48de.camel@neuling.org>
From:   Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Date:   Fri, 7 Jun 2019 08:47:47 +0530
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On 6/7/19 6:20 AM, Michael Neuling wrote:
> On Thu, 2019-06-06 at 12:59 +0530, 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>
> 
> How long has this been around? Should we be CCing stable?

"bl .save_nvgprs" was added in the commit 5aae8a5370802 ("powerpc, hw_breakpoints:
Implement hw_breakpoints for 64-bit server processors"), which was merged in
v2.6.36.