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 <mrat...@linux.microsoft.com> Sent: Wednesday, September 3, >>> 2025 7:17 PM >>>> >>>> On 9/2/25 07:42, Michael Kelley wrote: >>>>> From: Mukesh Rathor <mrat...@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.
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. Thanks, -Mukesh
