.. SPDX-License-Identifier: GPL-2.0
I/O request handling
====================
An I/O request of a User VM, which is constructed by the hypervisor, is
distributed by the ACRN Hypervisor Service Module to an I/O client
corresponding to the address range of the I/O request. Details of I/O request
handling are described in the following sections.
1. I/O request
--------------
For each User VM, there is a shared 4-KByte memory region used for I/O requests
communication between the hypervisor and Service VM. An I/O request is a
256-byte structure buffer, which is 'struct acrn_io_request', that is filled by
an I/O handler of the hypervisor when a trapped I/O access happens in a User
VM. ACRN userspace in the Service VM first allocates a 4-KByte page and passes
the GPA (Guest Physical Address) of the buffer to the hypervisor. The buffer is
used as an array of 16 I/O request slots with each I/O request slot being 256
bytes. This array is indexed by vCPU ID.
2. I/O clients
--------------
An I/O client is responsible for handling User VM I/O requests whose accessed
GPA falls in a certain range. Multiple I/O clients can be associated with each
User VM. There is a special client associated with each User VM, called the
default client, that handles all I/O requests that do not fit into the range of
any other clients. The ACRN userspace acts as the default client for each User
VM.
Below illustration shows the relationship between I/O requests shared buffer,
I/O requests and I/O clients.
::
+------------------------------------------------------+
| Service VM |
|+--------------------------------------------------+ |
|| +----------------------------------------+ | |
|| | shared page ACRN userspace | | |
|| | +-----------------+ +------------+ | | |
|| +----+->| acrn_io_request |<-+ default | | | |
|| | | | +-----------------+ | I/O client | | | |
|| | | | | ... | +------------+ | | |
|| | | | +-----------------+ | | |
|| | +-|--------------------------------------+ | |
||---|----|-----------------------------------------| |
|| | | kernel | |
|| | | +----------------------+ | |
|| | | | +-------------+ HSM | | |
|| | +--------------+ | | | |
|| | | | I/O clients | | | |
|| | | | | | | |
|| | | +-------------+ | | |
|| | +----------------------+ | |
|+---|----------------------------------------------+ |
+----|-------------------------------------------------+
|
+----|-------------------------------------------------+
| +-+-----------+ |
| | I/O handler | ACRN Hypervisor |
| +-------------+ |
+------------------------------------------------------+
3. I/O request state transition
-------------------------------
The state transitions of an ACRN I/O request are as follows.
::
FREE -> PENDING -> PROCESSING -> COMPLETE -> FREE -> ...
- FREE: this I/O request slot is empty
- PENDING: a valid I/O request is pending in this slot
- PROCESSING: the I/O request is being processed
- COMPLETE: the I/O request has been processed
An I/O request in COMPLETE or FREE state is owned by the hypervisor. HSM and
ACRN userspace are in charge of processing the others.
4. Processing flow of I/O requests
----------------------------------
a. The I/O handler of the hypervisor will fill an I/O request with PENDING
state when a trapped I/O access happens in a User VM.
b. The hypervisor makes an upcall, which is a notification interrupt, to
the Service VM.
c. The upcall handler schedules a worker to dispatch I/O requests.
d. The worker looks for the PENDING I/O requests, assigns them to different
registered clients based on the address of the I/O accesses, updates
their state to PROCESSING, and notifies the corresponding client to handle.
e. The notified client handles the assigned I/O requests.
f. The HSM updates I/O requests states to COMPLETE and notifies the hypervisor
of the completion via hypercalls.
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