Re: [PATCH v5 08/10] x86: hyperv: Add mshv_handler irq handler and setup function

[Nuno Das Neves <nunodasneves@linux.microsoft.com>]

On 3/7/2025 9:38 AM, Michael Kelley wrote:
> From: Nuno Das Neves <nunodasneves@linux.microsoft.com> Sent: Friday, February 28, 2025 4:38 PM
>>
>> On 2/26/2025 3:43 PM, Stanislav Kinsburskii wrote:
>>> On Wed, Feb 26, 2025 at 03:08:02PM -0800, Nuno Das Neves wrote:
>>>> This will handle SYNIC interrupts such as intercepts, doorbells, and
>>>> scheduling messages intended for the mshv driver.
>>>>
>>>> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
>>>> Reviewed-by: Wei Liu <wei.liu@kernel.org>
>>>> Reviewed-by: Tianyu Lan <tiala@microsoft.com>
>>>> ---
>>>>  arch/x86/kernel/cpu/mshyperv.c | 9 +++++++++
>>>>  drivers/hv/hv_common.c         | 5 +++++
>>>>  include/asm-generic/mshyperv.h | 1 +
>>>>  3 files changed, 15 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
>>>> index 0116d0e96ef9..616e9a5d77b4 100644
>>>> --- a/arch/x86/kernel/cpu/mshyperv.c
>>>> +++ b/arch/x86/kernel/cpu/mshyperv.c
>>>> @@ -107,6 +107,7 @@ void hv_set_msr(unsigned int reg, u64 value)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(hv_set_msr);
>>>>
>>>> +static void (*mshv_handler)(void);
>>>>  static void (*vmbus_handler)(void);
>>>>  static void (*hv_stimer0_handler)(void);
>>>>  static void (*hv_kexec_handler)(void);
>>>> @@ -117,6 +118,9 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
>>>>  	struct pt_regs *old_regs = set_irq_regs(regs);
>>>>
>>>>  	inc_irq_stat(irq_hv_callback_count);
>>>> +	if (mshv_handler)
>>>> +		mshv_handler();
>>>
>>> Can mshv_handler be defined as a weak symbol doing nothing instead
>>> of defining it a null pointer?
>>> This should allow to simplify this code and get rid of
>>> hv_setup_mshv_handler, which looks redundant.
>>>
>> Interesting, I tested this and it does seems to work! It seems like
>> a good change, thanks.
> 
> Just be a bit careful. When CONFIG_HYPERV=n, mshyperv.c still gets
> built even through none of the other Hyper-V related files do.  There
> are #ifdef CONFIG_HYPERV in mshyperv.c to eliminate references to
> Hyper-V files that wouldn't be built. I'd suggest doing a test build with
> that configuration to make sure it's all clean.
> 
Thanks Michael - I don't think it would be an issue since the __weak version
would be defined in mshyperv.c itself, replacing the function pointer.

However, I went and tested this __weak version again with CONFIG_MSHV_ROOT=m
and it does not actually work. Everything seems ok at first (it compiles,
can insert the module), but upon starting a guest, the interrupts don't get
delivered to the root (or rather, they don't get handled by mshv_hander()).

This seems to match with what the ld docs say - There's an option
LD_DYNAMIC_LINK to allow __weak symbols to be overridden by the dynamic
linker, but this is not enabled in the kernel.

So I will stick with the current implementation.

Nuno

> Michael
> 
>>
>>> Reviewed-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
>>>
>>>> +
>>>>  	if (vmbus_handler)
>>>>  		vmbus_handler();
>>>>
>>>> @@ -126,6 +130,11 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
>>>>  	set_irq_regs(old_regs);
>>>>  }
>>>>
>>>> +void hv_setup_mshv_handler(void (*handler)(void))
>>>> +{
>>>> +	mshv_handler = handler;
>>>> +}
>>>> +
>>>>  void hv_setup_vmbus_handler(void (*handler)(void))
>>>>  {
>>>>  	vmbus_handler = handler;
>>>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>>>> index 2763cb6d3678..f5a07fd9a03b 100644
>>>> --- a/drivers/hv/hv_common.c
>>>> +++ b/drivers/hv/hv_common.c
>>>> @@ -677,6 +677,11 @@ void __weak hv_remove_vmbus_handler(void)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(hv_remove_vmbus_handler);
>>>>
>>>> +void __weak hv_setup_mshv_handler(void (*handler)(void))
>>>> +{
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(hv_setup_mshv_handler);
>>>> +
>>>>  void __weak hv_setup_kexec_handler(void (*handler)(void))
>>>>  {
>>>>  }
>>>> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
>>>> index 1f46d19a16aa..a05f12e63ccd 100644
>>>> --- a/include/asm-generic/mshyperv.h
>>>> +++ b/include/asm-generic/mshyperv.h
>>>> @@ -208,6 +208,7 @@ void hv_setup_kexec_handler(void (*handler)(void));
>>>>  void hv_remove_kexec_handler(void);
>>>>  void hv_setup_crash_handler(void (*handler)(struct pt_regs *regs));
>>>>  void hv_remove_crash_handler(void);
>>>> +void hv_setup_mshv_handler(void (*handler)(void));
>>>>
>>>>  extern int vmbus_interrupt;
>>>>  extern int vmbus_irq;
>>>> --
>>>> 2.34.1
>>>>
>