Re: [Xen-devel] Correct DTB compatibility for xen /hypervisor node?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 31 Jan 2013, Ian Campbell wrote:
> On Thu, 2013-01-31 at 11:53 +0000, Stefano Stabellini wrote:
> > On Thu, 31 Jan 2013, Ian Campbell wrote:
> > > On Wed, 2013-01-30 at 16:49 +0000, Stefano Stabellini wrote:
> > > > On Mon, 28 Jan 2013, Ian Campbell wrote:
> > > > > Looks like we have both 4.2 and 4.3 in use. Which should it be?
> > > > > 
> > > > > $ rgrep xen-4 arch/arm Documentation/
> > > > > arch/arm/boot/dts/xenvm-4.2.dts:              compatible = 
> > > > > "xen,xen-4.2", "xen,xen";
> > > > > Documentation/devicetree/bindings/arm/xen.txt:        compatible = 
> > > > > "xen,xen-4.3", "xen,xen";
> > > > > 
> > > > > FWIW I've always used 4.2 in my own DTB files.
> > > > > 
> > > > > Strictly speaking the API is the Xen 3.x API, but that doesn't seem
> > > > > terribly relevant.
> > > > 
> > > > I have always used 4.2 too.
> > > > Even though the one in xen.txt is just an example it might be worth
> > > > updating with xen-4.2 to avoid confusions.
> > > 
> > > Or maybe we should update to 4.3 everywhere.
> > > 
> > > > > I suppose arch_get_xen_caps ought to return something consistent?
> > > > 
> > > > Right.
> > > > 
> > > > From my point of view, it would be useful to return a string
> > > > that expresses the current hypervisor version (xen-4.3 from the Xen 4.3
> > > > release, xen-4.4 from the Xen 4.4 release and so on) rather than 
> > > > xen-3.0.
> > > > I would do the same in the compatibility node of the DTS.
> > > 
> > > Remember that the capability string returned by the hypervisor is not
> > > exposing the version of the hypervisor -- it is exposing the version of
> > > the Hypervisor *ABI*, which is a subtly different thing.
> > > 
> > > I think we should follow the hypervisor version until we freeze the ABI,
> > > at which point we hardcode whichever version we froze at?
> > 
> > That's reasonable, but we shouldn't be afraid of bumping that number.
> 
> Once we have frozen the ABI we should be very afraid of bumping that
> number, because it would indicate that old guests might not run...

OK.

In that case I suggest that we should NOT expose the hypercall ABI in
the compatible node of the DTS, but the hypervisor version.
After all, like you pointed out, the guest already has a way to get the
hypercall ABI version, it doesn't need two. It gives us more flexibility
and it is certainly going to turn out useful in the future.

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