[lkml]   [2005]   [Aug]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [PATCH] 3/5 explicit-iopl
    > Well... maybe.  On Opteron and/or Intel EMT it may not be a win.  The 
    > cost of the branch could overtake the cost of the POPF (that's the
    > expensive one). Grrr.

    Not on Opteron, but probably on Intel.

    iirc popf will actually flush parts of the trace cache, while
    a branch shouldn't do that. So I expect it to be a win.

    > >Can we perhaps get rid of the PUSHF/POPF in the SYSENTER syscall path too?
    > >iirc they were only for single stepping. But SYSENTER doesn't disable
    > >single stepping, so the debug handler could detect this and set
    > >some magic flag that restores it on syscall exit.
    > >
    > >
    > A context switch requires IRET, which requires the flags to be saved, so
    > you can't eliminate the pushf (*) IIRC, the popf is already omitted.

    If we have iopl and TF elsewhere then the flags could just be replaced
    with a default value.

    Drawback would be that it would be slightly incompatible to before,
    but then it's just a function call and function calls are not
    required to preserve flags.

    > (*) Well, you could. It's just that system calls would have to clobber
    > flags - hmm.. sysenter based calls already do. But I'm not 100% sure

    They do pushf.

    > there isn't some bogon case where kernel preemption could cause you a
    > problem. Keeping around the fake IRET frame still appears to be a good
    > thing to do just for the benefit of ptrace / debug functionality. PUSHF
    > is cheap on every core I have measured on.

    Are you sure? I thought it was expensive on Northwood at least.
    Haven't measured on a Prescott or newer.

    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    More majordomo info at
    Please read the FAQ at

     \ /
      Last update: 2005-08-04 17:55    [W:0.021 / U:120.220 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site