linux-kernel - Re: [RFC kgr on klp 4/9] livepatch: add kgr infrastructure

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Date:	Tue, 05 May 2015 15:27:19 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	live-patching@...r.kernel.org, jpoimboe@...hat.com,
	sjenning@...hat.com, jkosina@...e.cz, vojtech@...e.cz,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Miroslav Benes <mbenes@...e.cz>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-s390@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [RFC kgr on klp 4/9] livepatch: add kgr infrastructure

On 05/04/2015, 02:23 PM, Martin Schwidefsky wrote:
> On Mon,  4 May 2015 13:40:20 +0200
> Jiri Slaby <jslaby@...e.cz> wrote:
> 
>> This means:
>> * add a per-thread flag to indicate whether a task is in the old or in
>>   the new universe,
>> * reset it in _slow_ paths of syscall's entry/exit,
>> * add helpers around the flag to sched.h,
>> * export the status in /proc/<pid>/kgr_in_progress,
> 
>> @@ -217,6 +226,7 @@ ENTRY(system_call)
>>  	mvc	__PT_INT_CODE(4,%r11),__LC_SVC_ILC
>>  	stg	%r14,__PT_FLAGS(%r11)
>>  .Lsysc_do_svc:
>> +	HANDLE_KGRAFT %r12
>>  	lg	%r10,__TI_sysc_table(%r12)	# address of system call table
>>  	llgh	%r8,__PT_INT_CODE+2(%r11)
>>  	slag	%r8,%r8,2			# shift and test for svc 0
> 
> This is not the slow path, .Lsysc_do_svc is on the main svc path. It is
> "only" two instruction but nevertheless this should be avoided.

Hi,

the commit log says the reset is in the slow path, not the test. But OK,
we can optimize, see below.

> One way is to combine it with the _TIF_TRACE mechanics:
> 
> .Lsysc_nr_ok:
>         xc      __SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
>         stg     %r2,__PT_ORIG_GPR2(%r11)
>         stg     %r7,STACK_FRAME_OVERHEAD(%r15)
>         lgf     %r9,0(%r8,%r10)                 # get system call add.
>  ->     tm      __TI_flags+6(%r12),_TIF_TRACE>>8
>  ->     jnz     .Lsysc_tracesys
>         basr    %r14,%r9                        # call sys_xxxx
>         stg     %r2,__PT_R2(%r11)               # store return value
> 
> Add _TIF_KGR_IN_PROGRESS to _TIF_TRACE and branch to a new label,
> e.g. to .Lsysc_trace. Distinguish between _TIF_KGR_IN_PROGRESS and
> the other trace reasons and either call s390_handle_kgraft or
> do_syscall_trace_enter / do_syscall_trace_exit.
> 
> The same for the exit work, add _TIF_KGR_IN_PROGRESS to _TIF_WORK
> and sort out the reason in .Lsysc_work. That avoids another two
> instructions on the main system call path.

I considered this, but there was no space in the word.

_TIF_WORK is:
TIF_NOTIFY_RESUME       0
TIF_SIGPENDING          1
TIF_NEED_RESCHED        2
TIF_UPROBE              7

_TIF_TRACE is:
TIF_SYSCALL_TRACE       3
TIF_SYSCALL_AUDIT       4
TIF_SECCOMP             5
TIF_SYSCALL_TRACEPOINT  6

=====

What I could do is to split them and make this setup:

_TIF_WORK:
TIF_NOTIFY_RESUME       0
TIF_SIGPENDING          1
TIF_NEED_RESCHED        2
TIF_KGR_IN_PROGRESS_W   3
TIF_UPROBE              7

_TIF_TRACE:
TIF_SYSCALL_TRACE       24
TIF_SYSCALL_AUDIT       25
TIF_SECCOMP             26
TIF_SYSCALL_TRACEPOINT  27
TIF_KGR_IN_PROGRESS_T   28

=====

Then make TIF_KGR_IN_PROGRESS_W fire when "tm"-ing _TIF_WORK in
"__TI_flags+7". TIF_KGR_IN_PROGRESS_T will work along with _TIF_TRACE
using "tm" on "__TI_flags+4".

What do you think?

thanks,
-- 
js
suse labs
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