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. >>> >>> 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
