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

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Date:	Mon, 17 Dec 2012 13:05:47 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"H. Peter Anvin" <hpa@...or.com>, 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

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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