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

[Mukesh R <mrathor@linux.microsoft.com>]

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.
>>>
>>> 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.
>>
>> 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."

Thanks,
-Mukesh