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

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Date:	Mon, 17 Dec 2012 07:21:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pavel Emelyanov <xemul@...allels.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	aarcange@...hat.com, ak@...ux.intel.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

Because it is almost impossible to do right?

Pavel Emelyanov <xemul@...allels.com> wrote:

>On 12/14/2012 10:44 PM, Andy Lutomirski wrote:
>> On Fri, Dec 14, 2012 at 10:35 AM, H. Peter Anvin <hpa@...or.com>
>wrote:
>>> On 12/14/2012 12:34 AM, Pavel Emelyanov wrote:
>>>> On 12/14/2012 06:20 AM, Andy Lutomirski wrote:
>>>>> On Thu, Dec 13, 2012 at 6:18 PM, H. Peter Anvin <hpa@...or.com>
>wrote:
>>>>>> Wouldn't the vdso get mapped already and could be mremap()'d.  If
>we
>>>>> really need more control I'd almost push for a device/filesystem
>node
>>>>> that could be mmapped the usual way.
>>>>>
>>>>> Hmm.  That may work, but it'll still break ABI.  I'm not sure that
>>>>> criu is stable enough yet that we should care.  Criu people?
>>>>
>>>> It's not yet, but we'd still appreciate the criu-friendly vdso
>redesign.
>>>>
>>>>> (In brief summary: how annoying would it be if the vdso was no
>longer
>>>>> just a bunch of constant bytes that lived somewhere?)
>>>>
>>>> It depends on what vdso is going to be. In the perfect case it
>should
>>>> a) be mremap-able to any address (or be at fixed address _forever_,
>but
>>>>    I assume this is not feasible);
>>>> b) have entry points at fixed (or somehow movable) places.
>>>>
>>>> I admit that I didn't understand your question properly, if I did,
>>>> please correct me.
>>>>
>>>
>>> mremap() should work.  At the same time, the code itself is not
>going to
>>> have any stability guarantees between kernel versions -- it
>obviously
>>> cannot.
>> 
>> We could guarantee that the symbols in the vdso resolve to particular
>> offsets within the vdso.  (Yes, this is ugly.)
>> 
>> Does criu support checkpointing with one version of a shared library
>> and restoring with another?
>
>No, neither we have this in plans.
>However, if somebody needs this and implements -- why not?!
>
>Thanks,
>Pavel

-- 
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