Re: [PATCH v6 13/21] gunyah: vm_mgr: Introduce basic VM Manager

[Elliot Berman <quic_eberman@quicinc.com>]

Hi Arnd, Greg,

On 11/4/2022 9:19 PM, Trilok Soni wrote:
> On 11/4/2022 3:38 PM, Elliot Berman wrote:
>>
>>
>> On 11/4/2022 1:10 AM, Arnd Bergmann wrote:
>>> On Fri, Nov 4, 2022, at 01:11, Elliot Berman wrote:
>>>> On 11/2/2022 5:24 PM, Greg Kroah-Hartman wrote:
>>>>> On Wed, Nov 02, 2022 at 11:45:12AM -0700, Elliot Berman wrote:
>>>>>
>>>>> Even if you don't support it 1:1, at least for the ones that are the
>>>>> same thing, pick the same numbers as that's a nicer thing to do, 
>>>>> right?
>>>>>
>>>>
>>>> Does same thing == interpretation of arguments is the same? For
>>>> instance, GH_CREATE_VM and KVM_CREATE_VM interpret the arguments
>>>> differently. Same for KVM_SET_USERSPACE_MEMORY. The high level
>>>> functionality should be similar for most all hypervisors since they 
>>>> will
>>>> all support creating a VM and probably sharing memory with that VM. The
>>>> arguments for that will necessarily look similar, but they will 
>>>> probably
>>>> be subtly different because the hypervisors support different features.
>>>
>>> I think in the ideal case, you should make the arguments and the
>>> command codes the same for any command where that is possible. If
>>> you come across a command that is shared with KVM but just needs
>>> another flag, that would involve coordinating with the KVM maintainers
>>> about sharing the definition so the same flag does not get reused
>>> in an incompatible way.
>>>
>>
>> I think the converse also needs to be true; KVM would need to check that
>> new flags don't get used in some incompatible way with Gunyah, even if
>> one of us is just -EINVAL'ing. I don't think Gunyah and KVM should be
>> reliant on the other reviewing shared ioctls.
>>
>> The problem is a bit worse because "machine type" is architecture-
>> dependent whereas the planned Gunyah flags are architecture-independent.
>> KVM within itself re-uses flags between architectures so Gunyah would
>> need to reserve some flags from all architectures that KVM supports.
> 
> I agree w/ Elliot. We would like to keep Gunyah independent and not rely 
> on the existing KVM ioctls space. We should allow new hypervisor drivers 
> interfaces addition in Linux kernel without them relying on KVM.
> 
>>
>>> For commands that cannot fit into the existing definition, there
>>> should be a different command code, using your own namespace and
>>> not the 0xAE block that KVM has. It still makes sense to follow
>>> the argument structure roughly here, unless there is a technical
>>> reason for making it different.
>>>
>>>> I don't think userspace that supports both KVM and Gunyah will benefit
>>>> much from re-using the same numbers since those re-used ioctl calls
>>>> still need to sit within the context of a Gunyah VM.
>>>
>>> One immediate benefit is for tools that work on running processes,
>>> such as strace, gdb or qemu-user. If they encounter a known command,
>>> they can correctly display the arguments etc.
>>>
>>
>> We can update these tools and anyway there will be different ioctls to
>> get started. There are important ioctls that wouldn't be correctly
>> displayed off the bat anyway; work would need to be done to support the
>> Gunyah ioctls either way. Whereas tooling update is temporary, the
>> coupling of KVM and Gunyah ioctls would be permanent.
> 
> Agree, tools can be updated and that is the easy part as we grow the s/w 
> stack around Gunyah in userspace, like we already do w/ CrosVM (Virtual 
> Machine Manager) and QEMU will be next followed by rust-vmm. All of them 
> can be done without Gunyah ioctls relying anything on the KVM ioctls. 
> Elliot has also explained very well that we don't to go to KVM 
> maintainers for any of our additions and we also don't want them to come 
> to us, since there is no interoperability testing. It is best that both 
> Hypervisors and their Linux interfaces evolve independently.

Are above explanations reasonable to not re-use KVM ioctl numbers?

Thanks,
Elliot