Re: MISRA violations in hypercall-defs

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 9 Aug 2023, Julien Grall wrote:
> On 09/08/2023 23:06, Stefano Stabellini wrote:
> > On Wed, 9 Aug 2023, Julien Grall wrote:
> > > Hi,
> > > 
> > > On 09/08/2023 21:35, Stefano Stabellini wrote:
> > > > P.S.
> > > > Julien, Bertrand, do you think we should unsupport (in SUPPORT.md, today
> > > > it is not clarified) 32-bit guests on a 64-bit ARM hypervisor?
> > > 
> > > I read your explanation above and I don't really understand why you would
> > > want
> > > to de-support it. This works pretty well and I am not aware of any issue
> > > right
> > > now to run 32-bit guest on 64-bit HW.
> > 
> > I am happy either way. The reason why I brought it up is that we don't
> > have a specific test for this in gitlab-ci
> 
> But a gitlab CI test can be added, right? I mean it would seem to be odd to
> use this as a justification because a lot of features (e.g. passthrough,
> suspend/resume...) would end up to be de-support it as gitlab CI is still in
> early stage.

Yes that's fair

> > and Jan raised concerns that
> > greater-than 32-bit values as possible as ret from hypercalls on a
> > 64-bit build of Xen.
> 
> This is a known problem and it was discussed several times on the ML in the
> past years.
> 
> There is a theorical problem because in theory all the hypercalls could return
> a value that can't fit in 32-bit.
> 
> However, AFAIK, only the memory hypercall XENMEM_maximum_gpfn may return a
> 64-bit value on 64-bit Xen.
> 
> It is not a problem for a 32-bit domain issues the hypercall on itself because
> the guest physical maximum address should never be greater than 40-bit (so
> 28-bit page frame number) and therefore could fit in 32-bit.
> 
> The only problem is if you want to use a 32-bit toolstack on 64-bit. But Jan
> sent a patch for SUPPORT.md to clarify this is not meant to always work (see
> [1]).
> 
> Please let me know if you are aware of any other truncations.

No, that's fine. This is exactly what I wanted to know :-)