Add flexible syscall interface

1 files changed, 62 insertions, 0 deletions

diff --git a/microkernel/mach/gnumach/projects/mach_5.mdwn b/microkernel/mach/gnumach/projects/mach_5.mdwn
index 0dc63228..bf678556 100644
--- a/microkernel/mach/gnumach/projects/mach_5.mdwn
+++ b/microkernel/mach/gnumach/projects/mach_5.mdwn
@@ -40,6 +40,8 @@ using an union.  The kernel indicates this using a distinct message
 type.  MIG-generated code will detect this, and do the receiver lookup
 using a specialized translation function.
 
+### Status
+
 This change has been implemented in GNU Mach and MIG 1.5.
 
 ## Type descriptor rework
@@ -70,3 +72,63 @@ The Mach4 type descriptor contains one unused bit.  This bit can be
 used to indicate that this message uses a Mach5 style index.  MIG can
 be modified to handle both cases for a smooth transition to the new
 ABI.
+
+### Status
+
+Not started.
+
+## Flexible syscall interface
+
+Currently, the GNU Mach kernel uses trap gates to enter the kernel (on
+i386).  We always suspected this mechanism to be slow, but afaik noone
+quantified that.
+
+Tl;dr: sysenter is twice as fast as a trap gate (on my system).
+
+I have a prototype that allows one to enter the kernel using sysenter.
+Here are the numbers:
+
+    start sysenter: mach_print using [trap gate] [sysenter].
+    Running 268435456(1U<<28) times mach_print("")...
+      using trap gate:  45s960000us 171.214342ns    5840632.202 (1/s)
+       using sysenter:  20s600000us 76.740980ns     13030847.379 (1/s)
+    Running 268435456(1U<<28) times mach_msg (NULL, ...)...
+     using glibc stub:  46s050000us 171.549618ns    5829217.286 (1/s)
+      using trap gate:  44s820000us 166.967511ns    5989189.112 (1/s)
+       using sysenter:  20s050000us 74.692070ns     13388302.045 (1/s)
+    exiting.
+
+So using sysenter is roughly 95ns faster.  To put this into
+perspective, sending a simple (ie. no ports/external data in body)
+message takes ~950ns on my system.  That suggests that merely using
+sysenter improves our IPC performance by ~10%.
+
+### Implementation
+
+I'd like to implement something similar:
+
+1. There is a platform dependent way to map a special page.
+2. That page contains a function that executes a syscall.
+
+This way we do not hardcode the system call method into the ABI.  The
+kernel selects one appropriate for the processor, and we are free to
+change this interface anytime we want.
+
+### Required ABI changes
+
+None.  We merely provide another way to call the kernel on existing
+platforms.
+
+On i386, the 'platform dependent way' to get the syscall wrapper is to
+use the current syscall mechanism to map a special device (the
+"syscall" device, or "/dev/syscall" on the Hurd) similar to how the
+mapped time interface works.
+
+### Status
+
+A prototype exists.
+
+### Discussions
+
+* <https://lists.gnu.org/archive/html/bug-hurd/2015-05/msg00000.html>
+