add multiple times opening support to a virtserialport

[Matt Ma]

Hi all,

Linaro has developed the foundation for the new Android Emulator code
base based on a fairly recent upstream QEMU code base, when we
re-based the code, we updated the device model to be more virtio based
(for example the drives are now virtio block devices). The aim of this
is to minimise the delta between upstream and the Android specific
changes to QEMU. One Android emulator specific feature is the
AndroidPipe.

AndroidPipe is a communication channel between the guest system and
the emulator itself. Guest side device node can be opened by multi
processes at the same time with different service name. It has a
de-multiplexer on the QEMU side to figure out which service the guest
actually wanted, so the first write after opening device node is the
service name guest wanted, after QEMU backend receive this service
name, create a corresponding communication channel, initialize related
component, such as file descriptor which connect to the host socket
serve. So each opening in guest will create a separated communication
channel.

We can create a separate device for each service type, however some
services, such as the OpenGL emulation, need to have multiple open
channels at a time. This is currently not possible using the
virtserialport which can only be opened once.

Current virtserialport can not  be opened by multiple processes at the
same time. I know virtserialport has provided buffers beforehand to
cache data from host to guest, so even there is no guest read, data
can still be transported from host to guest kernel, when there is
guest read request, just copy cached data to user space.

We are not sure clearly whether virtio can support
multi-open-per-device semantics or not, followings are just our
initial ideas about adding multi-open-per-device feature to a port:

* when there is a open request on a port, kernel will allocate a
portclient with new id and __wait_queue_head to track this request
* save this portclient in file->private_data
* guest kernel pass this portclient info to QEMU and notify that the
port has been opened
* QEMU backend will create a clientinfo struct to track this
communication channel, initialize related component
* we may change the kernel side strategy of allocating receiving
buffers in advance to a new strategy, that is when there is a read
request:
    - allocate a port_buffer, put user space buffer address to
port_buffer.buf, share memory to avoid memcpy
    - put both portclient id(or portclient addrss) and port_buffer.buf
to virtqueue, that is the length of buffers chain is 2
    - kick to notify QEMU backend to consume read buffer
    - QEMU backend read portclient info firstly to find the correct
clientinfo, then read host data directly into virtqueue buffer to
avoid memcpy
    - guest kernel will wait(similarly in block mode, because the user
space address has been put into virtqueue) until QEMU backend has
consumed buffer(all data/part data/nothing have been sent to host
side)
    - if nothing has been read from host and file descriptor is in
block mode, read request will wait through __wait_queue_head until
host side is readable

* above read logic may change the current behavior of transferring
data to guest kernel even without guest user read

* when there is a write request:
    - allocate a port_buffer, put user space buffer address to
port_buffer.buf, share memory to avoid memcpy
    - put both portclient id(or portclient addrss) and port_buffer.buf
to virtqueue, the length of buffers chain is 2
    - kick to notify QEMU backend to consume write buffer
    - QEMU backend read portclient info firstly to find the correct
clientinfo, then write the virtqueue buffer content to host side as
current logic
    - guest kernel will wait(similarly in block mode, because the user
space address has been put into virtqueue) until QEMU backend has
consumed buffer(all data/part data/nothing have been receive from host
side)
    - if nothing has been sent out and file descriptor is in block
mode, write request will wait through __wait_queue_head until host
side is writable

We obviously don't want to regress existing virtio behaviour and
performance and welcome the communities expertise to point out
anything we may have missed out before we get to far down implementing
our initial proof-of-concept.

Thanks,
Matt Ma
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