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    SubjectRe: FOR REVIEW: New x86-64 vsyscall vgetcpu()
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    On Friday 16 June 2006 17:31, 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?

    On x86-64 it's just %fs:offset. gcc is a bit dumb on this and usually
    loads the base address from %fs:0 first.

    >
    > > 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.

    Your scheme relies on task_struct fields being on a known offset
    in the page. But slab cache coloring varies the offset to make the data
    spread out better in the caches.

    > 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.

    It varies relative to the start of page.

    That was one of the bigger wins relative to the task_struct in stack
    page of 2.4 had.

    >
    > > Also you would make task_struct part of the userland ABI which
    > > seems like a very very bad idea to me. It means we couldn't change
    > > it anymore.
    >
    > We can make some wrapper, e.g.:
    >
    > user_per_cpu_var(name, offset)

    You would need to wrap everything and likely users would like
    task_struct so much that they accessed it anyways without your wrappers.

    > "vgetcpu()" would also be added to the ABI which we couldn't change
    > easily either.

    Yes, but it's a defined function. No different from a system call.

    -Andi

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