Re: [Qemu-devel] [PATCH v5 2/2] mach-virt: Provide sample configuration files

[Andrew Jones]

On Thu, Feb 09, 2017 at 04:10:43PM +0100, Andrea Bolognani wrote:
> On Thu, 2017-02-09 at 13:28 +0100, Andrew Jones wrote:
> > > Please keep in mind that I want to be able
> > > to use the very same paragraph both for q35 and mach-virt.
> > 
> > I'm not sure having the exact same paragraph is a reasonable
> > goal.
> 
> If not the exact same, there is no reason for it not to
> be at least 90% shareable IMHO.
> 
> > Does q35 have platform devices like mach-virt?
> 
> I don't know. And I'm not sure we need to talk about them
> or their aarch64 counterparts at all in the sample
> configuration file, because AFAIK with the exception of the
> GIC version they are not user configurable, or at the very
> least not something the user will actively want to
> configure (correct me if I'm wrong).

I'm OK with not mentioning them "too much" in this file. I
agree with Peter though that to be more accurate we shouldn't
state that '-nodefaults' results in a machine with nothing.
Therefore, some wording that states -nodefaults ensures no
non-builtin peripherals are automatically added, but builtin
peripherals, such as the PL011, will remain, seems like a
good idea.

> 
> > The
> > devices we get on a 'qemu -nodefaults -machine virt' instance
> > are MMIO driven devices on the board, rather than devices
> > hung off the pcie host bridge.
> 
> We want people to use PCIe instead of MMIO, though, so
> again why should we even mention those?

We want to use virtio-pci devices, as opposed to virtio-mmio
devices, which means using the PCIe ports you add vs. the
virtio-mmio transports. That doesn't mean we don't still
want to use the MMIO devices builtin to the board though,
the PL011 (UART) being a prime example.

> 
> It's just a sample configuration file we're talking about,
> not a complete reference of the aarch64 architecture, so
> it's entirely okay to skimp on details that don't directly
> impact most users and be opinionated.

I agree with being skimpy, but we don't want to cross the line
into misleading.

> 
> > Perhaps we can list the uart as an example; I don't suppose
> > it'll ever be removed. If we use wording such as "such as",
> > then it allows expansion to the board without a commitment to
> > update the list.
> > 
> > BTW, when I stated "-nodefaults provides us a base mach-virt
> > board with no peripherals.", I meant no _additional_
> > peripherals plugged into the board's virtio-mmio transports,
> > nor hung off the host bridge. Is there a word for those?
> > Maybe just non-builtin peripherals?
> 
> Please propose the alternative wording you'd like to see
> so we can discuss it :)

I guess I did above with "... -nodefaults ensures no non-builtin
peripherals are automatically added, but builtin peripherals,
such as the PL011, will remain..."

Thanks,
drew