Re: [Qemu-devel] [RFC 0/9] Introduce light weight PC platform pc-lite

[Paolo Bonzini]

On 17/06/2016 10:14, Chao Peng wrote:
> Basically:
> - it removes old ISA devices and support only PCI devices;

I think you need to keep at least the RTC, otherwise where does Linux
get the time of day from?

> - it removes 8259, instead use MSI as much as possible. IOAPIC and PCI
>   PIN are still kept to support ACPI SCI;
> - it supports PCIE ( you can use MMFG instead of 0xcf8/0xcfc port
>   access);
> - it gets rid of legacy firmware interfaces and supports ACPI tables;
> - it supports CPU/memory/PCI hotplug;
> - it supports Linux-guest only at present;
> - You may need carefully configure guest kernel;
> - You are forced to use virtio-serial-pci, old 8250/16550 is not there;

It doesn't support PCIe hotplug though, I think? (Because it doesn't
support PCI bridges and PCIe hotplug doesn't work for root complex
devices).  So is it ACPI-based hotplug?

Lack of 8250/16550 means lack of earlyprintk.  I know the driver is slow
though, so I understand that.

Anyway, I guess all the items above are acceptable.

The ones that I think are "less acceptable" are just two. :)

1) I am a bit worried about introducing a custom northbridge and PM
device.  In principle you could remove most ISA devices (especially
those that take long to initialize) and the 8259 while keeping Q35 and
ICH9.  This would make it easier to choose between having a firmware and
direct guest kernel load.

In general I'd model the lightweight devices around Q35 and ICH9, not
PIIX.  ICH9 in particular is good because it integrates the PM device
and it has an ISA bridge for the RTC and perhaps an optional 8250.  But
it would be even better to use Q35 and ICH9, not model around them. :)

2) this:

> - it loads guest kernel directly, no BIOS, no bootloader, no realmode
>   code;

... which is related to Linux-only support.  How much does this gain
over a minimal firmware (either SeaBIOS with the fw_cfg DMA interface,
or qboot with cbfs in parallel flash)?


> - it supports KVM-host only at present;

Do you know why?

Paolo