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    SubjectRe: Intel P6 vs P7 system call performance
    Linus Torvalds wrote:
    >>
    >>Destroying %ecx is a lot less destructive than destroying %eip and %esp...
    >
    > Actually, as far as the kernel is concerned, they are about equally bad.
    >

    Right, but from a user-mode point of view it means at least one extra
    instruction.

    > Destroying %eip is the _least_ bad register to destroy, since the kernel
    > can control that part, and it is trivial to just have a single call site.

    Trivial, perhaps, but it requires a call/ret pair in userspace, which is
    a fairly expensive form of push/pop.

    > The good news is that since both of them suck, it's easier to make the
    > six-argument decision. Since six arguments are problematic for all major
    > "fast" system calls, my executive decision is to just say that
    > six-argument system calls will just have to continue using the old and
    > slower system call interface. It's kind of a crock, but it's simply due to
    > silly CPU designers.

    Oh, so you're not going to do the "read from stack" thing? (Agreed, by
    the way, on the CPU design -- both SYSENTER and SYSCALL suck. SYSCALL
    was changed rather substantially in x86-64 for that reason.)

    -hpa



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