lkml.org 
[lkml]   [2008]   [Jul]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Eric W. Biederman wrote:
> Nope. It achieves that affect with a magic set of relocations instead
> of linker magic.
>

Well, the code gcc generates for -fstack-protector emits a literal
"%gs:40", so there's no relocations at all.

>> 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.
>>
>
> As a practical matter I like that approach (except for extra code size
> of the offsets).
>

Yes, and there's no reason we couldn't do the same on 64-bit, aside from
the stack-protector's use of %gs:40. There's no code-size cost in large
offsets, since they're always 32-bits anyway (there's no short absolute
addressing mode).

>> 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.
>>
>
> Yes, I completely agree with that. It doesn't mean however that we
> can't keep gcc ignorant and generate the same code manually.
>

Yes, I see. I haven't looked at that specifically, but I think both
Rusty and Andi have, and it gets tricky with modules and -ve kernel
addresses, or something.

> Well I was thinking threads switching on a cpu having the kinds of problems you
> described when it was tried on ppc.

Uh, I think we're having a nomenclature imprecision here. Strictly
speaking, the kernel doesn't have threads, only tasks and CPUs. We only
care about per-cpu data, not per-task data, so the concern is not
"threads switching on a CPU" but "CPUs switching on (under) a task".
But I think we understand each other regardless ;)

If we manually generate %gs-relative references to percpu data, then
it's no different to what we do with 32-bit, whether it be a specific
symbol address or using the TLS relocations.

J


\
 
 \ /
  Last update: 2008-07-01 23:55    [W:0.100 / U:1.208 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site