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Understanding QEMU Devices (2018)

Here are some notes that may help newcomers understand what is actually happening with QEMU devices: With QEMU, one thing to remember is that we are trying to emulate what an Operating System (OS) would see on bare-metal hardware. Most bare-metal machines are basically giant memory maps, where software poking at a particular address will have a particular side effect (the most common side effect is, of course, accessing memory; but other common regions in memory include the register banks for controlling particular pieces of hardware, like the hard drive or a network card, or even the CPU itself).

Much of the speedups in virtio come by its design - the guest sets aside a portion of regular memory for the bulk of its command queue, and only has to kick a single register to then tell QEMU to read the command queue (fewer mapped register accesses mean fewer vmexits), coupled with handshaking guarantees that the guest driver won’t be changing the normal memory while QEMU is acting on it. So QEMU has, as one of its many network card emulations, an e1000 device, which is mapped to the same guest memory region as a real one would live on bare metal. And at the end of the day, all virtualization really means is running a particular set of assembly instructions (the guest OS) to manipulate locations within a giant memory map for causing a particular set of side effects, where QEMU is just a user-space application providing a memory map and mimicking the same side effects you would get when executing those guest instructions on the appropriate bare metal hardware.

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