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
<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
<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> 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
<youpi> perhaps it got dropped in 64bit mode