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    SubjectRe: [PATCH 1/5] Split wait_noreap_copyout()
    At Mon, 11 May 2009 14:17:08 +0200, Ingo Molnar wrote:

    > > if (put_user(signal, &infop->si_signo) ||
    > > put_user(0, &infop->si_errno) ||
    > > put_user((short)why, &infop->si_code) ||
    > > put_user(pid, &infop->si_pid) ||
    > > put_user(uid, &infop->si_uid) ||
    > > put_user(status, &infop->si_status))
    > > return -EFAULT;
    >
    > For best assembly code this should generally be written as a series
    > of:
    >
    > __uaccess_err |= __put_user(x, ptr);
    > __uaccess_err |= __put_user(y, ptr);
    > __uaccess_err |= __put_user(z, ptr);
    >
    > As this generates non-dependent, compressed, branch-less code.

    Yeah, my first intention was to eliminate a lot of branches in one
    place. It's terrible for CPU pipeline, I bet.

    > See the (new) put_user_try / put_user_ex() / put_user_catch()
    > abstraction in arch/x86/include/asm/uaccess.h, and how all the x86
    > signal code makes use of that to optimize such patterns of per field
    > user copies.

    So, there's catch block to handle GPF and the code inside of `try'
    block is still branch-less, right? I was thinking of minimized version
    of struct siginfo (up to si_uid) and copying it with single
    copy_to_user(), but the idea with try/catch is definitely much
    better.

    --
    wbr, Vitaly


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