linux-kernel - Re: [GIT pull] x86 vdso updates

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Message-ID: <BANLkTim-XGgkNGO+hAkQUsywqA5OKdjzcg@mail.gmail.com>
Date:	Sun, 29 May 2011 10:57:51 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	richard -rw- weinberger <richard.weinberger@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [GIT pull] x86 vdso updates

On Sun, May 29, 2011 at 5:51 AM, richard -rw- weinberger
<richard.weinberger@...il.com> wrote:
> On Fri, May 27, 2011 at 5:05 PM, Andrew Lutomirski <luto@....edu> wrote:
>> On Fri, May 27, 2011 at 10:59 AM, richard -rw- weinberger
>> <richard.weinberger@...il.com> wrote:
>>> On Fri, May 27, 2011 at 2:10 PM, Andrew Lutomirski <luto@....edu> wrote:
>>>> On Fri, May 27, 2011 at 7:59 AM, richard -rw- weinberger
>>>> If this is considered enough of a regression, then I guess we can
>>>> leave vsyscall64 around for awhile, but it will require extra work in
>>>> the soon-to-be syscall emulation hack to make sure that UML can still
>>>> trap the syscall.
>>>
>>> As long the time within UML is synchronous with the host everything is
>>> fine, right?
>>
>> I think so.  I haven't used UML in a long time.
>>
>>> So, as _last_ choice we could disable the ability to change the time within UML.
>>>
>>> IMHO it's not a big deal when getcpu() returns a wrong CPU layout on UML.
>>>
>>>> The real solution is to fix glibc to use the vDSO which should avoid
>>>> this problem entirely.
>>>
>
> Yesterday I had a closer look at 64bit UML.
> Glibc is always using vsyscalls because 64bit UML does not support the vDSO.
>
> On 32bit UML simply scans the ELF auxiliary vector provided by the host to
> get the address of the vDSO.
> How can I get this address on a 64bit host?

I believe it's exactly the same.  There's an auxv entry that points to the vDSO.

--Andy
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