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