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Android QEMU FAST PIPES
=======================
Introduction:
-------------
The Android emulator implements a special virtual device used to provide
_very_ fast communication channels between the guest system and the
emulator itself.
From the guest, usage is simply as follows:
1/ Open the /dev/qemu_pipe device for read+write
2/ Write a zero-terminated string describing which service you want to
connect.
3/ Simply use read() and write() to communicate with the service.
In other words:
fd = open("/dev/qemu_pipe", O_RDWR);
const char* pipeName = "<pipename>";
ret = write(fd, pipeName, strlen(pipeName)+1);
if (ret < 0) {
// error
}
... ready to go
Where <pipename> is the name of a specific emulator service you want to use.
Supported service names are listed later in this document.
Implementation details:
-----------------------
In the emulator source tree:
./hw/goldfish_pipe.c implements the virtual driver.
./hw/goldfish_pipe.h provides the interface that must be
implemented by any emulator pipe service.
./android/hw-pipe-net.c contains the implementation of the network pipe
services (i.e. 'tcp' and 'unix'). See below for details.
In the kernel source tree:
drivers/misc/qemupipe/qemu_pipe.c contains the driver source code
that will be accessible as /dev/qemu_pipe within the guest.
Device / Driver Protocol details:
---------------------------------
The device and driver use an I/O memory page and an IRQ to communicate.
- The driver writes to various I/O registers to send commands to the
device.
- The device raises an IRQ to instruct the driver that certain events
occured.
- The driver reads I/O registers to get the status of its latest command,
or the list of events that occured in case of interrupt.
Each opened file descriptor to /dev/qemu_pipe in the guest corresponds to a
32-bit 'channel' value allocated by the driver.
The following is a description of the various commands sent by the driver
to the device. Variable names beginning with REG_ correspond to 32-bit I/O
registers:
1/ Creating a new channel:
Used by the driver to indicate that the guest just opened /dev/qemu_pipe
that will be identified by a 32-bit value named '<channel>' here:
REG_CHANNEL = <channel>
REG_CMD = CMD_OPEN
IMPORTANT: <channel> should never be 0
2/ Closing a channel:
Used by the driver to indicate that the guest called 'close' on the
channel file descriptor.
REG_CHANNEL = <channel>
REG_CMD = CMD_CLOSE
3/ Writing data to the channel:
Corresponds to when the guest does a write() or writev() on the
channel's file descriptor. This command is used to send a single
memory buffer:
REG_CHANNEL = <channel>
REG_ADDRESS = <buffer-address>
REG_SIZE = <buffer-size>
REG_CMD = CMD_WRITE_BUFFER
status = REG_STATUS
NOTE: The <buffer-address> is the *GUEST* buffer address, not the
physical/kernel one.
IMPORTANT: The buffer sent through this command SHALL ALWAYS be entirely
contained inside a single page of guest memory. This is
enforced to simplify both the driver and the device.
When a write() spans several pages of guest memory, the
driver will issue several CMD_WRITE_BUFFER commands in
succession, transparently to the client.
The value returned by REG_STATUS should be:
> 0 The number of bytes that were written to the pipe
0 To indicate end-of-stream status
< 0 A negative error code (see below).
On important error code is PIPE_ERROR_AGAIN, used to indicate that
writes can't be performed yet. See CMD_WAKE_ON_WRITE for more.
4/ Reading data from the channel:
Corresponds to when the guest does a read() or readv() on the
channel's file descriptor.
REG_CHANNEL = <channel>
REG_ADDRESS = <buffer-address>
REG_SIZE = <buffer-size>
REG_CMD = CMD_READ_BUFFER
status = REG_STATUS
Same restrictions on buffer addresses/lengths and same set of error
codes.
5/ Waiting for write ability:
If CMD_WRITE_BUFFER returns PIPE_ERROR_AGAIN, and the file descriptor
is not in non-blocking mode, the driver must put the client task on a
wait queue until the pipe service can accept data again.
Before this, the driver will do:
REG_CHANNEL = <channel>
REG_CMD = CMD_WAKE_ON_WRITE
To indicate to the virtual device that it is waiting and should be woken
up when the pipe becomes writable again. How this is done is explained
later.
6/ Waiting for read ability:
This is the same than CMD_WAKE_ON_WRITE, but for readability instead.
REG_CHANNEL = <channel>
REG_CMD = CMD_WAKE_ON_WRITE
7/ Polling for write-able/read-able state:
The following command is used by the driver to implement the select(),
poll() and epoll() system calls where a pipe channel is involved.
REG_CHANNEL = <channel>
REG_CMD = CMD_POLL
mask = REG_STATUS
The mask value returned by REG_STATUS is a mix of bit-flags for
which events are available / have occured since the last call.
See PIPE_POLL_READ / PIPE_POLL_WRITE / PIPE_POLL_CLOSED.
8/ Signaling events to the driver:
The device can signal events to the driver by raising its IRQ.
The driver's interrupt handler will then have to read a list of
(channel,mask) pairs, terminated by a single 0 value for the channel.
In other words, the driver's interrupt handler will do:
for (;;) {
channel = REG_CHANNEL
if (channel == 0) // END OF LIST
break;
mask = REG_WAKES // BIT FLAGS OF EVENTS
... process events
}
The events reported through this list are simply:
PIPE_WAKE_READ :: the channel is now readable.
PIPE_WAKE_WRITE :: the channel is now writable.
PIPE_WAKE_CLOSED :: the pipe service closed the connection.
The PIPE_WAKE_READ and PIPE_WAKE_WRITE are only reported for a given
channel if CMD_WAKE_ON_READ or CMD_WAKE_ON_WRITE (respectively) were
issued for it.
PIPE_WAKE_CLOSED can be signaled at any time.
Available services:
-------------------
tcp:<port>
tcp:<hostname>:<port>
Open a TCP socket to a given port. This provides a very fast
pass-through that doesn't depend on the very slow internal emulator
NAT router. Note that you can only use the file descriptor with read()
and write() though, send() and recv() will return an ENOTSOCK error,
as well as any socket ioctl().
unix:<path>
Open a Unix-domain socket on the host.
opengles
Connects to the OpenGL ES emulation process. For now, the implementation
is equivalent to tcp:22468, but this may change in the future.
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