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Message-ID: <486A9D4F.8010508@goop.org>
Date: Tue, 01 Jul 2008 14:10:39 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: "Eric W. Biederman" <ebiederm@xmission.com>
CC: "H. Peter Anvin" <hpa@zytor.com>, Mike Travis <travis@sgi.com>,
       Christoph Lameter <clameter@sgi.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area
References: <20080604003018.538497000@polaris-admin.engr.sgi.com>	<485AC5D4.6040302@goop.org> <485ACA8F.10006@sgi.com>	<485ACD92.8050109@sgi.com> <485AD138.4010404@goop.org>	<485ADA12.5010505@sgi.com> <485ADC73.60009@goop.org>	<485BDB04.4090709@sgi.com> <485BE80E.10209@goop.org>	<Pine.LNX.4.64.0806201045460.15874@schroedinger.engr.sgi.com>	<485BF8F5.6010802@goop.org> <485BFFC5.6020404@sgi.com>	<m1skv7onvh.fsf@frodo.ebiederm.org> <486912C4.8070705@sgi.com>	<48691556.2080208@zytor.com> <48691E8B.4040605@sgi.com>	<m14p7asl3x.fsf@frodo.ebiederm.org> <48694B3B.3010600@goop.org>	<m1skuuov4t.fsf@frodo.ebiederm.org> <486A61A7.1000902@zytor.com>	<486A68DD.80702@goop.org> <m1d4lxgwxj.fsf@frodo.ebiederm.org>
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Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@goop.org> writes:
>
>   
>> H. Peter Anvin wrote:
>>     
>>> Eric W. Biederman wrote:
>>>       
>>>>> The zero-based PDA mechanism requires the introduction of a new ELF segment
>>>>> based at vaddr 0 which is sufficiently unusual that it wouldn't surprise me
>>>>> if
>>>>> its triggering some toolchain bug.
>>>>>           
>>>> Agreed.  Given the previous description my hunch is that the bug is occurring
>>>> during objcopy.  If vmlinux is good and the compressed kernel is bad.
>>>>
>>>>         
>>> Actually, it's not all that unusual... it's pretty common in various
>>> restricted environments.  That being said, it's probably uncommon for *64-bit*
>>> code.
>>>       
>> Well, it's also unusual because 1) it's vaddr 0, but paddr <high>, and 2) the
>> PHDRs are not sorted by vaddr order.  2) might actually be a bug.
>>     
>
> I just looked and gcc does not use this technique for thread local data.
>   

Which technique?  It does assume you put the thread-local data near %gs 
(%fs in userspace), and it uses a small offset (positive or negative) to 
reach it.

At present, the x86-64 only uses %gs-relative addressing to reach the 
pda, which are always small positive offsets.  It always accesses 
per-cpu data in a two-step process of getting the base of per-cpu data, 
then offsetting to find the particular variable.

x86-32 has no pda, and arranges %fs so that %fs:variable gets the percpu 
variant of variable.  The offsets are always quite large.

> My initial concern about all of this was not making symbols section relative
> is relieved as this all appears to be a 64bit arch thing where that doesn't
> matter.
>   

Why's that?  I thought you cared particularly about making the x86-64 
kernel relocatable for kdump, and that using non-absolute symbols was 
part of that?

> Has anyone investigated using the technique gcc uses for thread local storage?
> http://people.redhat.com/drepper/tls.pdf
>   

The powerpc guys tried using gcc-level thread-local storage, but it 
doesn't work well.  per-cpu data and per-thread data have different 
constraints, and its hard to tell gcc about them.  For example, if you 
have a section of preemptable code in your function, it's hard to tell 
gcc not to cache a "thread-local" variable across it, even though we 
could have switched CPUs in the meantime.

> In particular using the local exec model so we can say:
> movq %fs:x@tpoff,%rax
>
> To load the contents of a per cpu variable x into %rax ?
>
> If we can use that model it should make it easier to interface with things like
> the stack protector code.  Although we would still need to be very careful
> about thread switches.
>   

You mean cpu switches?  We don't really have a notion of thread-local 
data in the kernel, other than things hanging off the kernel stack.

    J
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