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

[Juergen Gross <jgross@xxxxxxxx>]

On 17.12.21 08:45, Jan Beulich wrote:
> On 17.12.2021 06:34, Juergen Gross wrote:
> > On 16.12.21 22:15, Stefano Stabellini wrote:
> > > 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:
> > > > > > 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 ?
> > Hence my suggestion to check the return value of _all_ hypercalls to be
> > proper sign extended int values for 32-bit guests. This would fix all
> > potential issues without silently returning truncated values.
> Are we absolutely certain we have no other paths left where a possibly
> large unsigned values might be returned? In fact while
> compat_memory_op() does the necessary saturation, I've never been fully
> convinced of this being the best way of dealing with things. The range
> of error indicators is much smaller than [-INT_MIN,-1], so almost
> double the range of effectively unsigned values could be passed back
> fine. (Obviously we can't change existing interfaces, so this mem-op
> will need to remain as is.)
In fact libxenctrl tries do deal with this fact by wrapping a memory_op
for a 32-bit environment into a multicall. This will work fine for a
32-bit Arm guest, as xen_ulong_t is a uint64 there.

So do_memory_op should return long on Arm, yes. OTOH doing so will
continue to be a problem in case a 32-bit guest doesn't use the
multicall technique for handling possible 64-bit return values.

So I continue to argue that on Arm the return value of a hypercall
should be tested to fit into 32 bits. The only really clean alternative
would be to have separate hypercall function classes for Arm 32- and
64-bit guests (which still could share most of the functions by letting
those return "int"). This would allow to use the 64-bit variant even for
32-bit guests in multicall (fine as the return field is 64-bit wide),
and a probably saturating compat version for the 32-bit guest direct
hypercall.

The needed adaptions in my series would be rather limited (an additional
column in the hypercall table, a split which macro to use in
do_trap_hypercall() on Arm depending on the bitness of the guest, the
addition of the Arm compat variant of do_memory_op()).

Thoughts?


Juergen

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