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



Hi,

On 18/12/2021 00:00, Stefano Stabellini wrote:
On Fri, 17 Dec 2021, Juergen Gross wrote:
On 17.12.21 11:41, Julien Grall wrote:
Hi Juergen,

On 17/12/2021 08:50, Juergen Gross wrote:
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.

It would make sense. But what would you return if the value doesn't fit?

I guess some errno value would be appropriate, like -EDOM, -ERANGE or
-E2BIG.

This seems to be better than the alternative below as it is a lot
simpler.

We would still need to special case XENMEM_maximum_reservation (or rework the implementation of the sub-op) because the value returned is an unsigned long. So technically, the unsigned value for -EDOM & co could be interpreted as the maximum host frame number.

I also would like to see the hypercall returning 'int' when they are only meant to return 32-bit value. This will make easier to spot someone that decide to return a 64-bit value.



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.

I am not entirely sure to understand this proposal. Can you clarify it?

1. In patch 5 modify the hypercall table by adding another column, so
    instead of:
    +table:           pv32     pv64     hvm32    hvm64    arm
    use:
    +table:           pv32     pv64     hvm32    hvm64    arm32    arm64

2. Let most of the hypercalls just return int instead of long:
    +rettype: do int

3. Have an explicit 64-bit variant of memory_op (the 32-bit one is the
    compat variant existing already):
    +rettype: do64 long
    +prefix: do64 PREFIX_hvm
    +memory_op(unsigned long cmd, void *arg)

4. Use the appropriate calls in each column:
    +memory_op         compat   do64     hvm      hvm      compat  do64

5. In the Arm hypercall trap handler do:
    if ( is_32bit_domain(current->domain) )
        call_handlers_arm32(...);
    else
        call_handlers_arm64(...);

6. In the multicall handler always do:
    call_handlers_arm64(...);
I am probably missing something. But why do we need to have separate call handlers for arm32/arm64?

Cheers,

--
Julien Grall



 


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