Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly

[Nuno Das Neves <nunodasneves@linux.microsoft.com>]

On 9/4/2025 11:18 AM, Mukesh R wrote:
> On 9/4/25 09:26, Michael Kelley wrote:
>> From: Mukesh R <mrathor@linux.microsoft.com> Sent: Wednesday, September 3, 2025 7:17 PM
>>>
>>> On 9/2/25 07:42, Michael Kelley wrote:
>>>> From: Mukesh Rathor <mrathor@linux.microsoft.com> Sent: Wednesday, August 27, 2025 6:00 PM
>>>>>
>>>>> At present, drivers/Makefile will subst =m to =y for CONFIG_HYPERV for hv
>>>>> subdir. Also, drivers/hv/Makefile replaces =m to =y to build in
>>>>> hv_common.c that is needed for the drivers. Moreover, vmbus driver is
>>>>> built if CONFIG_HYPER is set, either loadable or builtin.
>>>>>
>>>>> This is not a good approach. CONFIG_HYPERV is really an umbrella config that
>>>>> encompasses builtin code and various other things and not a dedicated config
>>>>> option for VMBUS. Vmbus should really have a config option just like
>>>>> CONFIG_HYPERV_BALLOON etc. This small series introduces CONFIG_HYPERV_VMBUS
>>>>> to build VMBUS driver and make that distinction explicit. With that
>>>>> CONFIG_HYPERV could be changed to bool.
>>>>
>>>> Separating the core hypervisor support (CONFIG_HYPERV) from the VMBus
>>>> support (CONFIG_HYPERV_VMBUS) makes sense to me. Overall the code
>>>> is already mostly in separate source files code, though there's some
>>>> entanglement in the handling of VMBus interrupts, which could be
>>>> improved later.
>>>>
>>>> However, I have a compatibility concern. Consider this scenario:
>>>>
>>>> 1) Assume running in a Hyper-V VM with a current Linux kernel version
>>>>     built with CONFIG_HYPERV=m.
>>>> 2) Grab a new version of kernel source code that contains this patch set.
>>>> 3) Run 'make olddefconfig' to create the .config file for the new kernel.
>>>> 4) Build the new kernel. This succeeds.
>>>> 5) Install and run the new kernel in the Hyper-V VM. This fails.
>>>>
>>>> The failure occurs because CONFIG_HYPERV=m is no longer legal,
>>>> so the .config file created in Step 3 has CONFIG_HYPERV=n. The
>>>> newly built kernel has no Hyper-V support and won't run in a
>>>> Hyper-V VM.

It surprises me a little that =m doesn't get 'fixed up' to =y in this case.
I guess any invalid value turns to =n, which makes sense most of the time.

>>>>
>>>> As a second issue, if in Step 1 the current kernel was built with
>>>> CONFIG_HYPERV=y, then the .config file for the new kernel will have
>>>> CONFIG_HYPERV=y, which is better. But CONFIG_HYPERV_VMBUS
>>>> defaults to 'n', so the new kernel doesn't have any VMBus drivers
>>>> and won't run in a typical Hyper-V VM.
>>>>
>>>> The second issue could be fixed by assigning CONFIG_HYPERV_VMBUS
>>>> a default value, such as whatever CONFIG_HYPERV is set to. But
>>>> I'm not sure how to fix the first issue, except by continuing to
>>>> allow CONFIG_HYPERV=m.

I'm wondering, is there a path for this change, then? Are there some
intermediate step/s we could take to minimize the problem?

>>>
>>> To certain extent, imo, users are expected to check config files
>>> for changes when moving to new versions/releases, so it would be a
>>> one time burden. 
>>
>> I'm not so sanguine about the impact. For those of us who work with
>> Hyper-V frequently, yes, it's probably not that big of an issue -- we can
>> figure it out. But a lot of Azure/Hyper-V users aren't that familiar with
>> the details of how the Kconfig files are put together. And the issue occurs
>> with no error messages that something has gone wrong in building
>> the kernel, except that it won't boot. Just running "make olddefconfig"
>> has worked in the past, so some users will be befuddled and end up
>> generating Azure support incidents. I also wonder about breaking
>> automated test suites for new kernels, as they are likely to be running
>> "make olddefconfig" or something similar as part of the automation.
>>
>>> CONFIG_HYPERV=m is just broken imo as one sees that
>>> in .config but magically symbols in drivers/hv are in kerenel.
>>>
>>
>> I agree that's not ideal. But note that some Hyper-V code and symbols
>> like ms_hyperv_init_platform() and related functions show up when
>> CONFIG_HYPERVISOR_GUEST=y, even if CONFIG_HYPERV=n. That's
>> the code in arch/x86/kernel/cpu/mshyperv.c and it's because Hyper-V
>> is one of the recognized and somewhat hardwired hypervisors (like
>> VMware, for example).
>>
>> Finally, there are about a dozen other places in the kernel that use
>> the same Makefile construct to make some code built-in even though
>> the CONFIG option is set to "m". That may not be enough occurrences
>> to make it standard practice, but Hyper-V guests are certainly not the
>> only case.
>>
>> In my mind, this is judgment call with no absolute right answer. What
>> do others think about the tradeoffs?
> 
> Wei had said in private message that he agrees this is a good idea. Nuno
> said earlier above: 
> 
> "FWIW I think it's a good idea, interested to hear what others think."
> 
That was before Michael pointed out the potential issues which I was
unaware of. Let's see if there's a path that is smoother for all the
downstream users who may be compiling with CONFIG_HYPERV=m.

Nuno

> Thanks,> -Mukesh
> 
>