Re: [PATCH v4 0/8] Introduce the microvm machine type

[Sergio Lopez <slp@redhat.com>]

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Christian Borntraeger <borntraeger@de.ibm.com> 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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