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.
Cheers,
--
Julien Grall
