[PATCH v10 10/26] gunyah: vm_mgr: Introduce basic VM Manager

[Arnd Bergmann]

On Fri, Feb 24, 2023, at 11:29, Srinivas Kandagatla wrote:
> On 23/02/2023 22:40, Elliot Berman wrote:

>>>> Does this means adding #define GH_VM_DEFAULT_ARG 0 ? I am not sure 
>>>> yet what arguments to add here.
>>>>
>>>> The ABI can add new "long" values to GH_CREATE_VM and that wouldn't 
>>>
>>> Sorry, that is exactly what we want to avoid, we can not change the 
>>> UAPI its going to break the userspace.
>>>
>>>> break compatibility with old kernels; old kernels reject it as -EINVAL.
>>>
>>> If you have userspace built with older kernel headers then that will 
>>> break. Am not sure about old-kernels.
>>>
>>> What exactly is the argument that you want to add to GH_CREATE_VM?
>>>
>>> If you want to keep GH_CREATE_VM with no arguments that is fine but 
>>> remove the conflicting comments in the code and document so that its 
>>> not misleading readers/reviewers that the UAPI is going to be modified 
>>> in near future.
>>>
>>>
>> 
>> The convention followed here comes from KVM_CREATE_VM. Is this ioctl 
>> considered bad example?
>> 
>
> It is recommended to only use _IO for commands without arguments, and 
> use pointers for passing data. Even though _IO can indicate either 
> commands with no argument or passing an integer value instead of a 
> pointer. Am really not sure how this works in compat case.
>
> Am sure there are tricks that can be done with just using _IO() macro 
> (ex vfio), but this does not mean that we should not use _IOW to be more 
> explicit on the type and size of argument that we are expecting.
>
> On the other hand If its really not possible to change this IOCTL to 
> _IOW and argument that you are referring would be with in integer range, 
> then what you have with _IO macro should work.

Passing an 'unsigned long' value instead of a pointer is fine for compat
mode, as a 32-bit compat_ulong_t always fits inside of the 64-bit
unsigned long. The downside is that portable code cannot have a
single ioctl handler function that takes both commands with pointers
and other commands with integer arguments, as some architectures
(i.e. s390, possibly arm64+morello in the future) need to mangle
pointer arguments using compat_ptr() but must not do that on integer
arguments.

     Arnd