VirtualBox Guest Additions: Difference between revisions
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Communication between the guest and the host happens through packets in memory. These can be anywhere in physical memory, and are relatively short. The MMIO operations that read or populate packets are synchronous, so setting up one page to pass back and forth is feasible. The basic process for sending a message to the VM is to prepare a packet and then write its (physical) address to the MMIO port. Receiving works the same way, as you must prepare a packet with a request type and provide its physical address; after the MMIO port write finishes, the packet will be populated with the appropriate values if the request was successful. Communication from the VM is primarily initiated by an IRQ on the PCI device's interrupt line, which should then be followed up by appropriate packet requests, one of which should be of a special "Acknowledge Events" type.
This page contains some struct definitions which could alternatively be obtained from headers provided by VirtualBox, but those headers are rather heavy and assume they are being used alongside a robust set of system headers, so instead we will define these structs ourselves.
Pseudo-code examples in the following sections will assume the availability of these functions. Adjust them to match your environment.▼
▲Pseudo-code examples in the following sections will assume the availability of these functions
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Mouse Integration provides mouse position information using an absolute coordinate system. It does not provide information on mouse buttons, though, and that continues to go through the standard PS/2 (or USB) mouse devices. If your OS supports USB devices, mouse integration can implemented through a USB tablet devices instead of the mechanism described in this article.
Mouse Integration operates entirely over the guest device. Once enabled, mouse movements are sent to the guest through mouse packet requests and a corresponding interrupt. It is important to note that the format of the coordinates in these packets is based on a range from 0 to 0xFFFF which needs to be scaled to the display resolution. Why this approach was taken over using actual pixel coordinates (scared or otherwise) is unknown.
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