Message-ID: <b74712b4-056c-b307-8f85-04241cfd7a57@linux.microsoft.com>
Date: Fri, 5 Sep 2025 17:34:04 -0700
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
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Subject: Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly

On 9/5/25 14:41, Mukesh R wrote:
> On 9/5/25 13:08, Nuno Das Neves wrote:
>> On 9/4/2025 11:18 AM, Mukesh R wrote:
>>> On 9/4/25 09:26, Michael Kelley wrote:
>>>> From: Mukesh R <mrathor@...ux.microsoft.com> Sent: Wednesday, September 3, 2025 7:17 PM
>>>>>
>>>>> On 9/2/25 07:42, Michael Kelley wrote:
>>>>>> From: Mukesh Rathor <mrathor@...ux.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.
> 
> Ok, we've already thought of it for sometime and not able to come up
> with any. IMO, it's a minor hickup, not major. This is stalling
> upcoming iommu and other patches which will use CONFIG_HYPERV and 
> add more dependencies, and it would be much harder to straighten 
> out then. So I hope you guys can come up with some solution sooner than
> later, I can't think of any.

Played around a bit, setting it to "default HYPERV" like Michael suggested
gives it a default value of y. I thought it fail if one is bool and other
is tristate. So that should help with transition.

Thanks,
-Mukesh

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