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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 16 Dec 2021, Stefano Stabellini wrote:
> 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?

I can see now that you meant to say that, no matter what is the max
pseudo-physical address supported by the VM, XENMEM_maximum_ram_page is
supposed to return the max memory page, which could go above the
addressibility limit of the VM.

So XENMEM_maximum_ram_page should potentially be able to return (1<<44)
even when called by an aarch32 VM, with max IPA 40-bit.

I would imagine it could be useful if dom0 is 32-bit but domUs are
64-bit on a 64-bit hypervisor (which I think it would be a very rare
configuration on ARM.)

Then it looks like XENMEM_maximum_ram_page needs to be able to return a
value > 32-bit when called by a 32-bit guest.

The hypercall ABI follows the ARM C calling convention, so a 64-bit
value should be returned using r0 and r1. But looking at
xen/arch/arm/traps.c:do_trap_hypercall, it doesn't seem it ever sets r1
today. Only r0 is set, so effectively we only support 32-bit return
values on aarch32 and for aarch32 guests.

In other words, today all hypercalls on ARM return 64-bit to 64-bit
guests and 32-bit to 32-bit guests. Which in the case of memory_op is
"technically" the correct thing to do because it matches the C
declaration in xen/include/xen/hypercall.h:

extern long
do_memory_op(
    unsigned long cmd,
    XEN_GUEST_HANDLE_PARAM(void) arg);

So...  I guess the conclusion is that on ARM do_memory_op should return
"long" although it is not actually enough for a correct implementation
of XENMEM_maximum_ram_page for aarch32 guests ?