1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
|
[[!meta copyright="Copyright © 2014, 2016 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.
[[!toc startlevel=2 levels=1]]
## 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>
* <https://lists.gnu.org/archive/html/bug-hurd/2016-09/msg00056.html>
## Interface for userspace drivers
We need to provide an interface suitable for implementing drivers in
userspace:
* A way to handle interrupts
* and a way to allocate memory suitable for DMA buffers
### Required ABI changes
None. This is a new interface. Debian/Hurd uses a non-standard rpc
id, so we do not change an existing procedure there.
### Status
A DDE-based solution is used in Debian/Hurd to provide network
drivers. A rump kernel prototype is implemented. These use a kernel
interface written by Zheng Da available in the
"master-user_level_drivers" branch in the GNU Mach repository.
### Discussions
* <https://lists.gnu.org/archive/html/bug-hurd/2016-02/msg00126.html>
|
