RE: [PATCH 03/11] Drivers: hv: Add support to setup percpu vmbus handler

Michael Kelley
2026-04-01T16:56:10Z

From: Naman Jain <namjain@linux.microsoft.com> Sent: Monday, March 16, 2026 5:13 AM
>
> Add a wrapper function - hv_setup_percpu_vmbus_handler(), similar to
> hv_setup_vmbus_handler() to allow setting up custom per-cpu VMBus
> interrupt handler. This is required for arm64 support, to be added
> in MSHV_VTL driver, where per-cpu VMBus interrupt handler will be
> set to mshv_vtl_vmbus_isr() for VTL2 (Virtual Trust Level 2).

Needing both hv_setup_vmbus_handler() and
hv_setup_percpu_vmbus_handler() seems unfortunate. Here's an
alternate approach to consider:

1. I think the x86 VMBus sysvec handler and the vmbus_percpu_isr()
functions could both use the same vmbus_handler global variable.
Looking at your changes in this patch set, hv_setup_vmbus_handler()
and hv_setup_percpu_vmbus_handler() are used together and always
set the same value.

2. So move the global variable vmbus_handler out from arch/x86
and into hv_common.c, and export it. The x86 sysvec handler can
still reference it, and vmbus_percpu_isr() in vmbus_drv.c can
also reference it. No need to have vmbus_percpu_isr() under
arch/arm64 or have a stub in hv_common.c.

3. hv_setup_vmbus_handler() and hv_remove_vmbus_handler()
also move to hv_common.c. The __weak stubs go away.

With these changes, only hv_setup_vmbus_handler() needs to
be called, and it works for both x86 with the sysvec handler and
for arm64 with vmbus_percpu_isr().

I haven't coded this up, so maybe there's some problematic detail,
but the idea seems like it would work. If it does work, some of my
comments below are no longer applicable.

>
> Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> ---
> arch/arm64/hyperv/mshyperv.c | 13 +++++++++++++
> drivers/hv/hv_common.c | 11 +++++++++++
> drivers/hv/vmbus_drv.c | 7 +------
> include/asm-generic/mshyperv.h | 3 +++
> 4 files changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index 4fdc26ade1d7..d4494ceeaad0 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -134,3 +134,16 @@ bool hv_is_hyperv_initialized(void)
> return hyperv_initialized;
> }
> EXPORT_SYMBOL_GPL(hv_is_hyperv_initialized);
> +
> +static void (*vmbus_percpu_handler)(void);
> +void hv_setup_percpu_vmbus_handler(void (*handler)(void))
> +{
> + vmbus_percpu_handler = handler;
> +}
> +
> +irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
> +{
> + if (vmbus_percpu_handler)
> + vmbus_percpu_handler();
> + return IRQ_HANDLED;
> +}
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index d1ebc0ebd08f..a5064f558bf6 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -759,6 +759,17 @@ void __weak hv_setup_vmbus_handler(void (*handler)(void))
> }
> EXPORT_SYMBOL_GPL(hv_setup_vmbus_handler);
>
> +irqreturn_t __weak vmbus_percpu_isr(int irq, void *dev_id)
> +{
> + return IRQ_HANDLED;
> +}
> +EXPORT_SYMBOL_GPL(vmbus_percpu_isr);
> +
> +void __weak hv_setup_percpu_vmbus_handler(void (*handler)(void))
> +{
> +}
> +EXPORT_SYMBOL_GPL(hv_setup_percpu_vmbus_handler);

You've implemented hv_setup_percpu_vmbus_handler() following
the pattern of hv_setup_vmbus_handler(), which is reasonable.
But that turns out to be unnecessarily complicated. The existing
hv_setup_vmbus_handler() has a portion in
arch/x86/kernel/cpu/mshyperv.c as a special case because it uses a
hard-coded interrupt vector on x86/x64, and has its own custom
sysvec code. And there's a need for a __weak stub in hv_common.c
so that vmbus_drv.c will compile on arm64.

But hv_setup_percpu_vmbus_handler() does not have the same
requirements. It could be implemented entirely in vmbus_drv.c,
with no code under arch/x86 or arch/arm64, and no __weak stub
in hv_common.c. vmbus_drv.c would just need to
EXPORT_SYMBOL_FOR_MODULES, like it already does with vmbus_isr.
I didn't code it up, but I think that approach would be simpler with
fewer piece-parts scattered all over. If so, it would be worth
breaking the symmetry with hv_setup_vmbus_handler().

> +
> void __weak hv_remove_vmbus_handler(void)
> {
> }
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index bc4fc1951ae1..f99d4f2d3862 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1413,12 +1413,6 @@ void vmbus_isr(void)
> }
> EXPORT_SYMBOL_FOR_MODULES(vmbus_isr, "mshv_vtl");
>
> -static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
> -{
> - vmbus_isr();
> - return IRQ_HANDLED;
> -}
> -
> static void vmbus_percpu_work(struct work_struct *work)
> {
> unsigned int cpu = smp_processor_id();
> @@ -1520,6 +1514,7 @@ static int vmbus_bus_init(void)
> if (vmbus_irq == -1) {
> hv_setup_vmbus_handler(vmbus_isr);
> } else {
> + hv_setup_percpu_vmbus_handler(vmbus_isr);
> ret = request_percpu_irq(vmbus_irq, vmbus_percpu_isr,
> "Hyper-V VMbus", &vmbus_evt);
> if (ret) {
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index 108f135d4fd9..b147a12085e4 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -22,6 +22,7 @@
> #include <linux/bitops.h>
> #include <acpi/acpi_numa.h>
> #include <linux/cpumask.h>
> +#include <linux/interrupt.h>
> #include <linux/nmi.h>
> #include <asm/ptrace.h>
> #include <hyperv/hvhdk.h>
> @@ -179,6 +180,8 @@ static inline u64 hv_generate_guest_id(u64 kernel_version)
>
> int hv_get_hypervisor_version(union hv_hypervisor_version_info *info);
>
> +irqreturn_t vmbus_percpu_isr(int irq, void *dev_id);
> +void hv_setup_percpu_vmbus_handler(void (*handler)(void));
> void hv_setup_vmbus_handler(void (*handler)(void));
> void hv_remove_vmbus_handler(void);
> void hv_setup_stimer0_handler(void (*handler)(void));
> --
> 2.43.0