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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 16 Dec 2021, Juergen Gross wrote:
> On 16.12.21 03:10, Stefano Stabellini wrote:
> > 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.
> 
> You are aware that this isn't the guest's max page, but the host's?
> 
> And all of this is about a 32-bit guest on a 64-bit host.

Yes, I am following, and it is always difficult to get the right
information out of the ARM Reference Manual, but from my search
ARMv8/aarch32 (which is 32-bit mode on 64-bit capable hardware) supports
max 40 bits.

>From G5.1.3 (ARM DDI 0487G.a):

---
For execution in AArch32 state, the Armv8 architecture supports:

- A VA space of up to 32 bits. The actual width is IMPLEMENTATION DEFINED .

- An IPA space of up to 40 bits. The translation tables and associated System 
registers define the width of the
implemented address space.
---

Julien, Bertrand, am I reading this right?