Re: [PATCH v3 06/25] docs: Xen ARM DT bindings

[Stefano Stabellini]

On Thu, 13 Sep 2012, Rob Herring wrote:
> On 09/12/2012 01:14 PM, Stefano Stabellini wrote:
> > On Wed, 12 Sep 2012, Stefano Stabellini wrote:
> >> On Tue, 28 Aug 2012, Rob Herring wrote:
> >>> You should look at ePAPR 1.1 which defines hypervisor related bindings.
> >>> While it is a PPC doc, we should reuse or extend what makes sense.
> >>>
> >>> See section 7.5:
> >>>
> >>> https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.1.pdf
> >>
> >> Thanks for the link, I wasn't aware of ePAPR.
> >>
> >> The hypervisor node defined by ePAPR is not very different from what I
> >> am proposing. Should I try to be compatible with the hypervisor
> >> specification above (as in compatible = "epapr,hypervisor-1.1")?
> >> Or should I just use it as a reference for my own specification?
> >>
> >> Personally I would rather avoid full compatibility with ePAPR.
> > 
> > In particular reading section 7.5, these are the required properties of
> > the ePAPR hypervisor node:
> > 
> > - compatible = "epapr,hypervisor-1.1"
> > compared to what I am proposing, it is laking information about what
> > hypervisor we are talking about (xen, kvm, vmware, etc) and the version
> > of the ABI (xen-4.2).
> 
> compatible properties are often changed. If we do deviate on the rest of
> the binding, then it needs to be different.
> 
> Turns out that powerpc KVM guests use "linux,kvm" and "epapr,hypervisor"
> doesn't even appear in the kernel.
> 
> We also perhaps have to consider the possibility of Xen on PowerPC. Then
> alignment is more important.

OK. In that case I think that we can just use "xen,xen-4.2".


> > - hcall-instructions
> > potentially interesting, but given that for Xen we are quite happy with
> > HVC, we are not going to add any secondary hypercall mechanisms,
> > therefore at the moment it would just result in a BUG if the specified
> > hcall instruction is != HVC. Besides if somebody else wanted to
> > implemented the Xen hypercall interface in a different way they could
> > just reimplement the hypercall wrappers, that would be easier than
> > trying to do it with this property.
> 
> It's really about the parameters passed with the HVC. The ePAPR may be a
> good model for defining this. Just grepping "hypervisor" under
> arch/powerpc, it's pretty clear hypervisor support happened first and
> the ePAPR came second. Hopefully we can avoid that for ARM.

Right. As I wrote in the other email, we could have a new property to
select the calling convention (and therefore the hypercall wrappers) to
be used with the hypervisor.


> > - guest-id
> > we usually make a point in trying not to tell the guest its own domid
> > because if the VM gets migrated to a different host it acquires a new
> > domid, therefore it should not rely on it.
> > 
> > - guest-name
> > we could pass the guest name here, but I don't see how it could be
> > of any use.
> > 
> 
> I have no issue with these being optional.

OK, good.


> > On the other hand, thinking more about what Xen needs in the device
> > tree, I think we could improve the current spec by clarifying the
> > meaning of the memory region and interrupt properties we currently
> > require. I thought about moving them to two separate children node with
> > an explicit name:
> > 
> > ---
> > 
> > * Xen hypervisor device tree bindings
> > 
> 
> I really think we need to define ARM hypervisor device tree bindings
> first, then Xen specific bindings as an addition to that. I worry that
> the KVM folks aren't paying attention and then want something different
> later on.

The problem is that there isn't much in common between Xen and KVM at
least from the DT point of view. I am not sure what would go into this
common hypervisor node.
The three key pieces of information that we are currently passing in the
DT (xen-4.2, a memory region, a PPI) are Xen specific.

If one day KVM (or another hypervisor vendor) decides to support the Xen
interface, can't they just have the Xen specific binding with a slightly
different compatible node?
For example:

compatible = "linux,kvm", "xen,xen-4.2"

wouldn't that mean "I am KVM but I can support the Xen interface"?


> > Xen ARM virtual platforms shall have the following properties and
> > children nodes:
> > 
> > - compatible property:
> > 	compatible = "xen,xen", "xen,xen-<version>";
> 
> "xen,xen" should be last as it is less specific.

Ah OK, thanks.


> >   where <version> is the version of the Xen ABI of the platform.
> > 
> > - grant_table child with the following properties:
> >     - name:
> > 	    name = "grant_table";
> 
> What's a grant table?

A Xen specific mechanism to share pages between guests.


> > 	- reg: specifies the base physical address and size of a region in
> > 	    memory where the grant table should be mapped to, using an
> >         HYPERVISOR_memory_op hypercall. 
> > 
> > - events child with the following properties:
> >     - name:
> >         name = "events";
> > 	- interrupts: the interrupt used by Xen to inject event notifications.
> 
> Why a child node? Just an interrupts property alone should be fine. If
> you have cases with different number of interrupts, the compatible
> property can distinguish that.

I see, that's a good point. In that case maybe it doesn't make sense to
move the memory region and the interrupt into two different children
nodes after all. I should probably leave the spec as it was.
 

> > 
> > 
> > Example:
> > hypervisor {
> > 		compatible = "xen,xen", "xen,xen-4.2";
> > 		#address-cells = <1>;
> > 		#size-cells = <1>;
> > 		#interrupt-cells = <3>;
> > 		ranges = <0xb0000000 0xb0000000 0x20000>;
> > 
> > 		grant_table {
> > 			name = "grant_table";
> > 			reg = <0xb0000000 0x20000>;
> > 		};
> > 
> > 		events {
> > 			name = "events";
> > 			interrupts = <1 15 0xf08>;
> > 		};
> > 	};
> > 
> 
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