On Fri, 27 Aug 2021, Oleksandr wrote:
> On 07.08.21 01:57, Julien Grall wrote:
> > Hi Stefano,
> > 
> > On 06/08/2021 23:26, Stefano Stabellini wrote:
> > > On Fri, 6 Aug 2021, Julien Grall wrote:
> > > > Hi Stefano,
> > > > 
> > > > On 06/08/2021 21:15, Stefano Stabellini wrote:
> > > > > On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
> > > > > > Hello, all.
> > > > > > 
> > > > > > I would like to clarify some bits regarding a possible update for
> > > > > > "Xen
> > > > > > device tree bindings for the guest" [1].
> > > > > > 
> > > > > > A bit of context:
> > > > > > We are considering extending "reg" property under the hypervisor
> > > > > > node and
> > > > > > we would like to avoid breaking backward compatibility.
> > > > > > So far, the "reg" was used to carry a single region for the grant
> > > > > > table
> > > > > > mapping only and it's size is quite small for the new improvement
> > > > > > we are currently working on.
> > > > > > 
> > > > > > What we want to do is to extend the current region [reg: 0] and add
> > > > > > an
> > > > > > extra regions [reg: 1-N] to be used as a safe address space for any
> > > > > > Xen specific mappings. But, we need to be careful about running
> > > > > > "new"
> > > > > > guests (with the improvement being built-in already) on "old" Xen
> > > > > > which is not aware of the extended regions, so we need the binding
> > > > > > to be
> > > > > > extended in a backward compatible way. In order to detect whether
> > > > > > we are running on top of the "new" Xen (and it provides us enough
> > > > > > space to
> > > > > > be used for improvement), we definitely need some sign to
> > > > > > indicate that.
> > > > > > 
> > > > > > Could you please clarify, how do you expect the binding to be
> > > > > > changed in
> > > > > > the backward compatible way?
> > > > > > - by adding an extra compatible (as it is a change of the binding
> > > > > > technically)
> > > > > > - by just adding new property (xen,***) to indicate that "reg"
> > > > > > contains
> > > > > > enough space
> > > > > > - other option
> > > > >    The current description is:
> > > > > 
> > > > > - 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 [...]
> > > > > 
> > > > > 
> > > > > Although it says "a region" I think that adding multiple ranges would
> > > > > be
> > > > > fine and shouldn't break backward compatibility.
> > > > > 
> > > > > In addition, the purpose of the region was described as "where the
> > > > > grant
> > > > > table should be mapped". In other words, it is a safe address range
> > > > > where the OS can map Xen special pages.
> > > > > 
> > > > > Your proposal is to extend the region to be bigger to allow the OS to
> > > > > map more Xen special pages. I think it is a natural extension to the
> > > > > binding, which should be backward compatible.
> > > > 
> > > > I agree that extending the reg (or even adding a second region) should
> > > > be fine
> > > > for older OS.
> > > > 
> > > > > 
> > > > > Rob, I am not sure what is commonly done in these cases. Maybe we just
> > > > > need an update to the description of the binding? I am also fine with
> > > > > adding a new compatible string if needed.
> > > > 
> > > > So the trouble is how a newer Linux version knows that the region is big
> > > > enough to deal with all the foreign/grant mapping?
> > > > 
> > > > If you run on older Xen, then the region will only be 16MB. This means
> > > > the
> > > > Linux will have to fallback on stealing RAM as it is today.
> > > > 
> > > > IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
> > > > ideally
> > > > want the OS to not fallback on stealing RAM (and close XSA-300). This is
> > > > where
> > > > we need a way to advertise it.
> > > > 
> > > > The question here is whether we want to use a property or a compatible
> > > > for
> > > > this.
> > > > 
> > > > I am leaning towards the latter because this is an extension of the
> > > > bindings.
> > > > However, I wasn't entirely whether this was a normal way to do it.
> 
> 
> May I please ask for the clarification how to properly advertise that we have
> extended region? By new compatible or property?

The current compatible string is defined as:

- compatible:
	compatible = "xen,xen-<version>", "xen,xen";
  where <version> is the version of the Xen ABI of the platform.


On the Xen side it is implemented as:

"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"


So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.


Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.

So one option is to rely on a check based on the Xen version. Example:

  version >= xen,xen-4.16


Or we need to go with a property. This seems safer and more solid. The
property could be as simple as "extended-region":

hypervisor {
	compatible = "xen,xen-4.16", "xen,xen";
    extended-region;
	reg = <0 0xb0000000 0 0x20000 0xc 0x0 0x1 0x0>;
	interrupts = <1 15 0xf08>;
};


Julien, do you have a better suggestion for the property name?