Re: [PATCH v3 02/13] xen: harmonize return types of hypercall handlers

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 15 Dec 2021, Juergen Gross wrote:
> On 14.12.21 18:36, Julien Grall wrote:
> > Hi,
> > 
> > On 08/12/2021 15:55, Juergen Gross wrote:
> > > Today most hypercall handlers have a return type of long, while the
> > > compat ones return an int. There are a few exceptions from that rule,
> > > however.
> > 
> > So on Arm64, I don't think you can make use of the full 64-bit because a
> > 32-bit domain would not be able to see the top 32-bit.
> > 
> > In fact, this could potentially cause us some trouble (see [1]) in Xen.
> > So it feels like the hypercalls should always return a 32-bit signed value
> > on Arm.
> 
> This would break hypercalls like XENMEM_maximum_ram_page which are able
> to return larger values, right?
> 
> > The other advantage is it would be clear that the top 32-bit are not
> > usuable. Stefano, what do you think?
> 
> Wouldn't it make more sense to check the return value to be a sign
> extended 32-bit value for 32-bit guests in do_trap_hypercall() instead?
> 
> The question is what to return if this is not the case. -EDOM?


I can see where Julien is coming from: we have been trying to keep the
arm32 and arm64 ABIs identical since the beginning of the project. So,
like Julien, my preference would be to always return 32-bit on ARM, both
aarch32 and aarch64. It would make things simple.

The case of XENMEM_maximum_ram_page is interesting but it is not a
problem in reality because the max physical address size is only 40-bit
for aarch32 guests, so 32-bit are always enough to return the highest
page in memory for 32-bit guests.

So XENMEM_maximum_ram_page could be the exception and return "long"
which is 4 bytes on aarch32 and 8 bytes on aarch64, and it is exactly
what we need.