Re: Fwd: 5-STABLE kernel build with icc broken

From: David Schultz (das_at_FreeBSD.ORG)
Date: 03/30/05

  • Next message: Ivan Voras: "MAC (was: A few thoughts...)" 
    Date: Wed, 30 Mar 2005 08:00:51 -0500
To: Peter Jeremy <PeterJeremy@optushome.com.au>

    On Wed, Mar 30, 2005, Peter Jeremy wrote:
    > On Tue, 2005-Mar-29 22:57:28 -0500, jason henson wrote:
    > >Later in that thread they discuss skipping the restore state to make
    > >things faster. The minimum buffer size they say this will be good for
    > >is between 2-4k. Does this make sense, or am I showing my ignorance?
    > >
    > >http://leaf.dragonflybsd.org/mailarchive/kernel/2004-04/msg00264.html
    >
    > Yes. There are a variety of options for saving/restoring FPU state:
    > a) save FPU state on kernel entry
    > b) save FPU state on a context switch (or if the kernel wants the FPU)
    > c) only save FPU state if a different process (or the kernel) wants the FPU
    > 1) restore FPU on kernel exit
    > 2) restore FPU state if a process wants the FPU.
    >
    > a and 1 are the most obvious - that's the way the integer registers are
    > handled.
    >
    > I thought FreeBSD used to be c2 but it seems it has been changed to b2
    > since I looked last.
    >
    > Based on the mail above, it looks like Dfly was changed from 1 to 2
    > (I'm not sure if it's 'a' or 'c' on save).
    >
    > On the i386 (and probably most other CPUs), you can place the FPU into
    > am "unavailable" state. This means that any attempt to use it will
    > trigger a trap. The kernel will then restore FPU state and return.
    > On a normal system call, if the FPU hasn't been used, the kernel will
    > see that it's still in an "unavailable" state and can avoid saving the
    > state. (On an i386, "unavailable" state is achieved by either setting
    > CR0_TS or CR0_EM). This means you avoid having to always restore FPU
    > state at the expense of an additional trap if the process actually
    > uses the FPU.

    This is basically what FreeBSD does on i386 and amd64. (As a
    disclaimer, I haven't read the code very carefully, so I might be
    missing some of the details.) Upon taking a trap for a process
    that has never used the FPU before, we save the FPU state for the
    last process to use the FPU, then load a fresh FPU state. On
    subsequent context switches, the FPU state for processes that have
    already used the FPU gets loaded before entering user mode, I
    think. I haven't studied the code in enough detail to know what
    happens for SMP, where a process could be scheduled on a different
    processor before its FPU state is saved on the first processor.
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  • Next message: Ivan Voras: "MAC (was: A few thoughts...)"

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