Christian Borntraeger <borntraeger@xxxxxxxxxx> writes: > On 24.09.19 14:44, Sergio Lopez wrote: >> Microvm is a machine type inspired by both NEMU and Firecracker, and >> constructed after the machine model implemented by the latter. >> >> It's main purpose is providing users a minimalist machine type free >> from the burden of legacy compatibility, serving as a stepping stone >> for future projects aiming at improving boot times, reducing the >> attack surface and slimming down QEMU's footprint. >> >> The microvm machine type supports the following devices: >> >> - ISA bus >> - i8259 PIC >> - LAPIC (implicit if using KVM) >> - IOAPIC (defaults to kernel_irqchip_split = true) >> - i8254 PIT >> - MC146818 RTC (optional) >> - kvmclock (if using KVM) >> - fw_cfg >> - One ISA serial port (optional) >> - Up to eight virtio-mmio devices (configured by the user) > > Just out of curiosity. > What is the reason for not going virtio-pci? Is the PCI bus really > that expensive and complicated? Well, expensive is a relative term. PCI does indeed require a significant amount of code and cycles, but that's for a good reason, as it provides an extensive bus logic allowing things like vector configuration, hot-plug, chaining, etc... On the other hand, MMIO lacks any kind of bus logic, as it basically works by saying "hey, take a look at this address, there may be something there" to the kernel, so of course is cheaper. This makes it ideal for microvm's aim of supporting a VM with the smallest amount of code, but bad for almost everything else. I don't think this means PCI is expensive. That would be the case if there were a bus providing similar functionality while requiring less code and cycles. And this is definitely not the case of MMIO. In other words, I think PCI cost is justified by its use case, while MMIO simplicity makes it ideal for some specific purposes (like microvm). Cheers, Sergio. > FWIW, I do not complain. When people start using virtio-mmio more > often this would also help virtio-ccw (which I am interested in) > as this forces people to think beyond virtio-pci.
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