Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly

[Mukesh R <mrathor@linux.microsoft.com>]

On 9/2/25 07:42, Michael Kelley wrote:
> From: Mukesh Rathor <mrathor@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.
> 
> 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. CONFIG_HYPERV=m is just broken imo as one sees that
in .config but magically symbols in drivers/hv are in kerenel.

Thanks,
-Mukesh


> See additional minor comments in Patches 1 and 2.
> 
> Michael
> 
>>
>> For now, hv_common.c is left as is to reduce conflicts for upcoming patches,
>> but once merges are mostly done, that and some others should be moved to
>> virt/hyperv directory.
>>
>> Mukesh Rathor (2):
>>   hyper-v: Add CONFIG_HYPERV_VMBUS option
>>   hyper-v: Make CONFIG_HYPERV bool
>>
>>  drivers/Makefile               |  2 +-
>>  drivers/gpu/drm/Kconfig        |  2 +-
>>  drivers/hid/Kconfig            |  2 +-
>>  drivers/hv/Kconfig             | 14 ++++++++++----
>>  drivers/hv/Makefile            |  4 ++--
>>  drivers/input/serio/Kconfig    |  4 ++--
>>  drivers/net/hyperv/Kconfig     |  2 +-
>>  drivers/pci/Kconfig            |  2 +-
>>  drivers/scsi/Kconfig           |  2 +-
>>  drivers/uio/Kconfig            |  2 +-
>>  drivers/video/fbdev/Kconfig    |  2 +-
>>  include/asm-generic/mshyperv.h |  8 +++++---
>>  net/vmw_vsock/Kconfig          |  2 +-
>>  13 files changed, 28 insertions(+), 20 deletions(-)
>>
>> --
>> 2.36.1.vfs.0.0
>>