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

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.)


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