Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

[Chase Venters]

On Fri, 16 Jun 2006, Zoltan Menyhart wrote:

> Andi Kleen wrote:
>
> >  That is not how user space TLS works. It usually has a base a register.
>
> Can you please give me a real life (simplified) example?
>
> >  This means it cannot be cache colored (because you would need a static
> >  offset) and you couldn't share task_structs on a page.
>
> I do not see the problem. Can you explain please?
> E.g. the scheduler pulls a task instead of the current one. The CPU
> will see "current->thread_info.cpu"-s of all the tasks at the same
> offset anyway.

Memory maps have to fall on page boundaries for lots of various reasons. 
Assuming a 16-word cache line, you've got plenty of spots you could align 
task_struct to within a page. (That number of spots is actually 
constrained by either sizeof(task_struct) or the number of colors).

The bottom line is that task_struct won't always be on a page boundary. If 
it's not on a page boundary in the physical page frames, it's not going to 
be on a page boundary in virtual memory either.

(Note also that if two task_structs shared a page, you'd have an 
information leak. I'm not sure with sizeof(task_struct) and cache 
alignment if task_structs are small enough for sharing, though. Definitely 
on hugepages.)

Thanks,
Chase
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