Re: [PATCH v2 13/13] SUPPORT.md: write down restriction of 32-bit tool stacks

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 15/07/2021 07:38, Jan Beulich wrote:
> On 14.07.2021 20:16, Julien Grall wrote:
> > On 05/07/2021 16:18, Jan Beulich wrote:
> > > Let's try to avoid giving the impression that 32-bit tool stacks are as
> > > capable as 64-bit ones.
> > Would you be able to provide a few examples of the known issues in the
> > commit message? This would be helpful for anyone to understand why we
> > decided to drop the support.
> Not sure how useful this is going to be.
It would at least be useful to me, so I can make an informed decision. I 
suspect it would also be for anyone reading it in the future. This is 
rather frustrating to find commit message with barely any rationale and 
no-one remembering why this was done...
I vaguely recall a discussion about 64-bit hypercall ([1]). I assume the 
decision to drop support is related to it, but I have no way to prove it 
from the commit message.
It is also not clear why adding the restriction is the way to go...

> This would be pointing at the
> declarations / definitions of various tool stack internal variables or
> structure fields. Which also is why ...
... is this because such issues are too widespread in libxc/libxl to fix 
it in long term?
> > At least on Arm, we tried to design the hypercall ABI in such a way that
> > it should be possible to use a 32-bit toolstack.
> ... keeping the ABI tidy in this regard didn't help at all (albeit it
> of course was a prereq to writing a tool stack that would be capable).
>
> Jan
[1] 
https://lore.kernel.org/xen-devel/71b8a4f1-9c18-36e7-56b1-3f1b1dabddd6@xxxxxxxx/
--
Julien Grall