GNU Hurd/ microkernel/ mach/ gnumach/ projects/ mach 5

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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.

  1. Protected payloads
  2. Type descriptor rework
  3. Flexible syscall interface
  4. Interface for userspace drivers

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

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