Re: device tree cpu node 'compatible' string when using KVM_ARM_PREFERRED_TARGET vcpu

[Christoffer Dall <christoffer.dall-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Mon, Nov 18, 2013 at 07:40:22PM +0000, Peter Maydell wrote:
> The KVM API has an ioctl KVM_ARM_PREFERRED_TARGET which we can use
> to create virtual CPUs of whatever the host CPU's "preferred" type
> is (typically "same as host"). This allows userspace to create a
> VCPU for best performance without having to have specific knowledge
> about that CPU.
> 
> However there's a slight wrinkle when userspace comes to create the
> device tree to pass to the guest kernel. The device tree binding
> for v7 says that the cpu nodes in the device tree must have a
> "compatible" string, and the only valid values it lists all nail
> things down to an exact CPU model. Requiring userspace to have
> an explicit mapping table of target values to dtb compat strings
> defeats the point of userspace not having to have hardcoded knowledge
> of all CPU types.
> 
> This problem doesn't exist for v8, because there the compat
> string is always "arm,arm-v8", indicating that the kernel
> doesn't care beyond "this is a v8 CPU of some kind".
> 
> In fact I'm told that for v7 the kernel doesn't care either (both
> because it needs to know about the CPU type much earlier than it
> has access to information in the dtb and also because this is
> all probeable via ID registers anyhow). So the obvious solution
> would be for userspace to pass a 'compatible' string "arm,arm-v7"
> (which the kernel will ignore anyway).
> 
> Does anybody see a problem with this approach? If it seems like a
> good plan, how do we go about getting it put into the official
> device tree binding docs?

I think it sounds like the only viable approach.

I guess we simply add it as an option to
Documentation/devicetree/bindings/arm/cpus.txt as a patch and send it to
lakml?  Am I missing some more difficult aspect here?  (assuming this
doesn't have to go through any DT stable tree thingy or anything like
that - it's a one-liner after all).

-Christoffer
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