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. Nuno > Thanks,> -Mukesh > >
