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devmem is used to represent MMIO devices like the boot ROM or a VESA framebuffer where doing a trap-and-emulate for every access is impractical. devmem is a hybrid of system memory (sysmem) and emulated device models. devmem is mapped in the guest address space via nested page tables similar to sysmem. However the address range where devmem is mapped may be changed by the guest at runtime (e.g. by reprogramming a PCI BAR). Also devmem is usually mapped RO or RW as compared to RWX mappings for sysmem. Each devmem segment is named (e.g. "bootrom") and this name is used to create a device node for the devmem segment (e.g. /dev/vmm/testvm.bootrom). The device node supports mmap(2) and this decouples the host mapping of devmem from its mapping in the guest address space (which can change). Reviewed by: tychon Discussed with: grehan Differential Revision: https://reviews.freebsd.org/D2762 MFC after: 4 weeks |
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