linux-kernel - Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit kernel

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Date:	Fri, 14 Dec 2012 14:48:50 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	Andy Lutomirski <luto@...capital.net>, aarcange@...hat.com,
	ak@...ux.intel.com, Pavel Emelyanov <xemul@...allels.com>,
	Stefani Seibold <stefani@...bold.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org, criu@...nvz.org, mingo@...hat.com,
	john.stultz@...aro.org, tglx@...utronix.de
Subject: Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit
 kernel

On 12/14/2012 02:43 PM, Cyrill Gorcunov wrote:
> On Fri, Dec 14, 2012 at 02:27:08PM -0800, H. Peter Anvin wrote:
>> On 12/14/2012 02:25 PM, Cyrill Gorcunov wrote:
>>>
>>> this would allow us to defer checkpoint until task finish vdso code. Peter,
>>> if I understand you correctly you propose we provide some own proxy-vdso
>>> which would redirect calls to real ones, right? But the main problem
>>> is that is exactly the idea to be able to c/r existing programs without
>>> recompiling and such (or I miss something here?).
>>
>> No, I'm proposing that you use a proxy-vdso which does nothing but
>> system calls, and therefore can be stable indefinitely.
> 
> This won't help in case of scenario you've been pointing in
> previous email (where c/r happens in a middle of vdso),
> would it? Because we still need somehow to be sure we're not
> checkpointing in a middle of signal handler which will return
> to some vdso place.

It is okay if and only if those vdso places never change... which I
think is doable if they only contain trival system call wrappers, i.e.
something like:

	movl $__SYS_gettimeofday, %eax
	syscall
	ret

These kinds of wrappers don't rely on live data provided by the kernel,
and so can be checkpointed together with the rest of the process.

	-hpa


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