[[!meta copyright="Copyright © 2011, 2012 Free Software Foundation, Inc."]]

[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
id="license" text="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
is included in the section entitled [[GNU Free Documentation
License|/fdl]]."]]"""]]

[[!tag open_issue_gnumach]]

[[!toc]]


# IRC, freenode, #hurd, 2011-05-07

    <braunr> very simple examples: system calls use old call gates, which are
      the slowest path to kernel space
    <braunr> modern processors have dedicated instructions now


# IRC, freenode, #hurd, 2012-04-22

    <braunr> rah: basically, system calls are slower on mach because they use
      call gates instead of newer sysenter/sysexit
    <youpi> braunr: sysenter/exit is a x86_64 thing
    <braunr> rah: apart from that, the code can't get much simpler, and *I*
      know, for i have studied it, and wrote a compatible version in a clone
      attempt
    <youpi> braunr: on a x86_64 port we'd probably use sysenter/exit
    <braunr> youpi: no there are 32-bits instructions, i don't remember if
      they're called sysenter, it's in my thesis though so i'm sure of it :)
    <youpi> braunr: ah, the other part
    <youpi> is linux-x86 using them?
    <braunr> youpi: yes, glibc uses them
    <youpi> and does it really change much nowadays?
    <youpi> what is the actual difference between int 80 and sysenter?
    <braunr> less checking
    <youpi> checking what?
    <youpi> the idt?
    <braunr> ring levels for example
    <youpi> well, checking a ring is fast :)
    <braunr> depending on the original and requested levels, there are lookups
      in tables
    <braunr> sysenter always assume 3 to 0 and 0 to 3 for sysexit
    <youpi> ah, also it assumes things about segments
    <youpi> so that indeed makes context things simpler
    <braunr> right
    <braunr> but mach doesn't uses int 0x80
    <braunr> it uses an lcall
    <braunr> which is a bit slower from what I could read some time ago
    <braunr> (not sure if it's still relevant)
    <youpi> actually in 64bit mode I had to catch lcall from the invalid
      instruction trap
    <youpi> perhaps it got dropped in 64bit mode