Re: [PATCH 06/11] Drivers: hv: Make sint vector architecture neutral in MSHV_VTL

[Naman Jain <namjain@linux.microsoft.com>]

On 4/13/2026 9:19 PM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Monday, April 13, 2026 4:48 AM
>>
>> On 4/1/2026 10:27 PM, Michael Kelley wrote:
>>> From: Naman Jain <namjain@linux.microsoft.com> Sent: Monday, March 16, 2026 5:13 AM
>>>>
>>>> Generalize Synthetic interrupt source vector (sint) to use
>>>> vmbus_interrupt variable instead, which automatically takes care of
>>>> architectures where HYPERVISOR_CALLBACK_VECTOR is not present (arm64).
>>>
>>> Sashiko AI raised an interesting question about the startup timing --
>>> whether the vmbus_platform_driver_probe() is guaranteed to have
>>> set vmbus_interrupt before the VTL functions below run and use it.
>>> What causes the mshv_vtl.ko module to be loaded, and hence run
>>> mshv_vtl_init()?
>>
>> There is no race condition here. The init ordering guarantees that
>> vmbus_interrupt is always set before mshv_vtl_synic_enable_regs()
>> reads it.
>>
>> The call chain for setting vmbus_interrupt:
>>
>>     subsys_initcall(hv_acpi_init)                          [level 4]
>>       -> platform_driver_register(&vmbus_platform_driver) and so on.
>>
>>
>> The call chain for reading vmbus_interrupt:
>>
>>     module_init(mshv_vtl_init)                             [level 6]
>>       -> hv_vtl_setup_synic()
>>         -> cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, ..., mshv_vtl_alloc_context, ...)
>>           -> mshv_vtl_alloc_context()
>>             -> mshv_vtl_synic_enable_regs()
>>               -> sint.vector = vmbus_interrupt
>>
>> do_initcalls() processes sections in order 0 through 7, so
>> hv_acpi_init() (level 4) is guaranteed to complete before
>> mshv_vtl_init() (level 6) runs.
>>
> 
> I think the situation is more complex than what you describe, depending
> on whether the VMBus driver and/or MSHV_VTL are built as modules vs.
> being built-in to the kernel image. In include/linux/module.h, see the
> comment for module_init() and how subsys_initcall() is mapped
> to module_init() when built as a module.
> 
> If both are built-in, then what you describe is correct. But if either or
> both are modules, then the respective init functions (hv_acpi_init
> and mshv_vtl_init) get called at the time the module is loaded, and
> not by do_initcalls(). I think hv_vmbus.ko gets loaded when an attempt
> is first made to access a disk, but I would need to look more closely to
> be sure. I don't have any understanding of what causes mshv_vtl.ko
> to be loaded. And what is the ordering if MSHV_VTL is built-in while
> VMBus is built as a module, or vice versa?
> 
> Michael
> 

Based on this, I still feel that this race is not possible.

hv_vmbus   mshv_vtl
    y          y  -> different initcall levels, no issues
    y          m  -> use without initialization is not possible
    m          y  -> config dependencies take care of this, and mshv_vtl 
is forced to compile as a module in this case.
    m          m  -> config and symbol dependencies should take care of 
it. mshv_vtl has symbol and config dependencies on hv_vmbus, and it 
won't allow loading mshv_vtl if hv_vmbus module is not loaded.

Relevant code here: kernel/module/main.c

Regards,
Naman