diff options
Diffstat (limited to 'microkernel')
| -rw-r--r-- | microkernel/mach/gnumach/projects.mdwn | 2 | ||||
| -rw-r--r-- | microkernel/mach/gnumach/projects/mach_5.mdwn | 138 |
2 files changed, 140 insertions, 0 deletions
diff --git a/microkernel/mach/gnumach/projects.mdwn b/microkernel/mach/gnumach/projects.mdwn index f4ef192a..62903fbe 100644 --- a/microkernel/mach/gnumach/projects.mdwn +++ b/microkernel/mach/gnumach/projects.mdwn @@ -35,6 +35,8 @@ so that no duplicate efforts end up. * [[Open Issues|tag/open_issue_gnumach]] + * [[Mach_5]] + * Update the core architecture and drivers * Check what NetBSD, FreeBSD and Linux do with their host specific code diff --git a/microkernel/mach/gnumach/projects/mach_5.mdwn b/microkernel/mach/gnumach/projects/mach_5.mdwn new file mode 100644 index 00000000..73aaef99 --- /dev/null +++ b/microkernel/mach/gnumach/projects/mach_5.mdwn @@ -0,0 +1,138 @@ +[[!meta copyright="Copyright © 2014 +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]] + +# The Mach5 proposal + +The Mach IPC mechanism is known to have deficiencies. Some of these +could be addressed with a new message ABI. A transition to 64-bit +architectures requires a new ABI definition anyway, so while we are at +it, we could straighten out some of these problems. + +This page is a place to keep track of such changes. + +## Protected payloads + +Protected payloads are a way of optimizing the receiver object lookup +in servers. A server may associate a payload with a receive right, +and any incoming message is tagged with it. The payload is an +pointer-wide unsigned integer, so the address of the associated server +side state can be used as payload. This removes the need for a hash +table lookup. + +### Required change to the message format + +Add a new field for the payload to the message header. + +### Implementation within the bounds of the Mach4 message format + +The payload can be provided in the same location as the local port +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 + +A Mach4 message body contains pairs of type descriptors and values. +Each type descriptor describes the kind and amount of data that +immediately follows in the message stream. As the kernel has to +rewrite rights and pointers to out-of-band memory, it has to parse the +message. As type information and values are interleaved, it has to +iterate over the whole message. + +Furthermore, there are two kinds of type descriptors, mach_msg_type_t +and mach_msg_type_long_t. The reason for this is that the amount of +data that can be described using mach_msg_type_t is just 131072 byte. +This is because msgt_size is an 8-bit value describing the size of one +element in bits, and msgt_number is an 12-bit value describing the +number of items. + +### Required change to the message format + +Group the type descriptors together at the beginning of the message to +provide an index into the data. Provide the element size in multiple +of the native word size avoiding the need for long type descriptors. + +### Implementation within the bounds of the Mach4 message format + +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 + +One trouble with sysenter/sysexit (or the amd equivalent) isn't +available on all processors. Linux solves this using the VDSO +mechanism. + +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> + |