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diff --git a/Documentation/virt/acrn/io-request.rst b/Documentation/virt/acrn/io-request.rst new file mode 100644 index 000000000000..6cc3ea0fa1f5 --- /dev/null +++ b/Documentation/virt/acrn/io-request.rst @@ -0,0 +1,97 @@ +.. 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. |