linux-kernel - Re: [V6,1/9] elf: Add new powerpc specifc core note sections

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Message-ID: <55349FD9.5040006@linux.vnet.ibm.com>
Date:	Mon, 20 Apr 2015 12:12:33 +0530
From:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:	Ulrich Weigand <Ulrich.Weigand@...ibm.com>
CC:	shuahkh@....samsung.com, Michael Neuling <mikey@...ling.org>,
	james.hogan@...tec.com, avagin@...nvz.org,
	Paul.Clothier@...tec.com, peterz@...radead.org, palves@...hat.com,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	dhowells@...hat.com, linuxppc-dev@...abs.org, kirjanov@...il.com,
	tglx@...utronix.de, oleg@...hat.com, davej@...hat.com,
	akpm@...ux-foundation.org, sukadev@...ux.vnet.ibm.com,
	Edjunior Barbosa Machado <emachado@...ux.vnet.ibm.com>,
	sam.bobroff@....ibm.com
Subject: Re: [V6,1/9] elf: Add new powerpc specifc core note sections

On 04/13/2015 02:18 PM, Anshuman Khandual wrote:
> On 04/10/2015 04:03 PM, Ulrich Weigand wrote:
>> Anshuman Khandual <khandual@...ux.vnet.ibm.com> wrote on 10.04.2015
>> 11:10:35:
> 
> I believed it stores the check pointed MSR value which was in the register
> before the transaction started. But then how it is different from the
> ckpt_regs.msr, I am not sure. Mikey or Michael should be able to clarify
> more on this. I can see "orig_msr" getting used in many places to hold the
> check pointed value of MSR.

tm_orig_msr is used during process context switch only. ckpt_regs
gets used in the signal context where we save all userspace context.
So ptrace would look into the saved MSR value correctly inside
ckpt_regs.msr slot. I believe thats the check pointed MSR value
we are interested in from the ptrace perspective not the tm_orig_msr
which just gets used during process context switch.


>>   I may be misreading kernel code, but it seems the kernel does not
>> actually
>>   use the ckpt_regs.msr slot at all, and therefore the corresponding slot
>> of
>>   the NT_PPC_TM_CGPR regset is likewise undefined/unused.  Would it not be
>>   more consistent to use that slot to pass the checkpointed MSR?
> 
> Hmm. Its a valid point. Would like to get some more clarity on this from
> Mikey whether that slot can be used for check pointed MSR value or not.
> Then why did we have these two slots to hold the same check pointed MSR
> value in the first place at all. Getting confused a bit.

Using ckpt_regs.msr during process context switch instead of tm_orig_msr seems
to be working fine and all the basic TM tests pass, in which case we can drop
tm_orig_msr from thread_struct. Will post a patch on this and see whats the
response from others.

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