Re: [PATCH v2] xen/arm: Convert runstate address during hypcall

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.07.2020 12:12, Julien Grall wrote:
> On 31/07/2020 07:39, Jan Beulich wrote:
>> We're fixing other issues without breaking the ABI. Where's the
>> problem of backporting the kernel side change (which I anticipate
>> to not be overly involved)?
> This means you can't take advantage of the runstage on existing Linux 
> without any modification.
> 
>> If the plan remains to be to make an ABI breaking change,
> 
> For a theoritical PoV, this is a ABI breakage. However, I fail to see 
> how the restrictions added would affect OSes at least on Arm.

"OSes" covering what? Just Linux?

> In particular, you can't change the VA -> PA on Arm without going 
> through an invalid mapping. So I wouldn't expect this to happen for the 
> runstate.
> 
> The only part that *may* be an issue is if the guest is registering the 
> runstate with an initially invalid VA. Although, I have yet to see that 
> in practice. Maybe you know?

I'm unaware of any such use, but this means close to nothing.

>>  then I
>> think this will need an explicit vote.
> 
> I was under the impression that the two Arm maintainers (Stefano and I) 
> already agreed with the approach here. Therefore, given the ABI breakage 
> is only affecting Arm, why would we need a vote?

The problem here is of conceptual nature: You're planning to
make the behavior of a common hypercall diverge between
architectures, and in a retroactive fashion. Imo that's nothing
we should do even for new hypercalls, if _at all_ avoidable. If
we allow this here, we'll have a precedent that people later
may (and based on my experience will, sooner or later) reference,
to get their own change justified.

Jan