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@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, September 3, 2025 7:17 PM >>>>> >>>>> On 9/2/25 07:42, Michael Kelley wrote: >>>>>> From: Mukesh Rathor <mrathor@xxxxxxxxxxxxxxxxxxx> 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
