[PATCH v2 07/15] arm64: hyperv: Add support for mshv_vtl_return_call

[Michael Kelley]

From: Naman Jain <namjain at linux.microsoft.com> Sent: Wednesday, April 29, 2026 2:57 AM
> 
> On 4/27/2026 11:08 AM, Michael Kelley wrote:
> > From: Naman Jain <namjain at linux.microsoft.com> Sent: Thursday, April 23, 2026 5:42 AM
> >>

[snip]

> >> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> >> index 08278547b84c..b4d80c9a673a 100644
> >> --- a/arch/x86/include/asm/mshyperv.h
> >> +++ b/arch/x86/include/asm/mshyperv.h
> >> @@ -286,7 +286,6 @@ struct mshv_vtl_cpu_context {
> >>   #ifdef CONFIG_HYPERV_VTL_MODE
> >>   void __init hv_vtl_init_platform(void);
> >>   int __init hv_vtl_early_init(void);
> >> -void mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0);
> >>   void mshv_vtl_return_call_init(u64 vtl_return_offset);
> >>   void mshv_vtl_return_hypercall(void);
> >>   void __mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0);
> >> @@ -294,7 +293,6 @@ int hv_vtl_get_set_reg(struct hv_register_assoc *regs, bool set, bool shared);
> >>   #else
> >>   static inline void __init hv_vtl_init_platform(void) {}
> >>   static inline int __init hv_vtl_early_init(void) { return 0; }
> >> -static inline void mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0) {}
> >>   static inline void mshv_vtl_return_call_init(u64 vtl_return_offset) {}
> >>   static inline void mshv_vtl_return_hypercall(void) {}
> >>   static inline void __mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0) {}
> >> diff --git a/drivers/hv/mshv_vtl.h b/drivers/hv/mshv_vtl.h
> >> index a6eea52f7aa2..103f07371f3f 100644
> >> --- a/drivers/hv/mshv_vtl.h
> >> +++ b/drivers/hv/mshv_vtl.h
> >> @@ -22,4 +22,7 @@ struct mshv_vtl_run {
> >>   	char vtl_ret_actions[MSHV_MAX_RUN_MSG_SIZE];
> >>   };
> >>
> >> +static_assert(sizeof(struct mshv_vtl_cpu_context) <= 1024,
> >> +	      "struct mshv_vtl_cpu_context exceeds reserved space in struct mshv_vtl_run");
> >> +
> >>   #endif /* _MSHV_VTL_H */
> >> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> >> index db183c8cfb95..8cdf2a9fbdfb 100644
> >> --- a/include/asm-generic/mshyperv.h
> >> +++ b/include/asm-generic/mshyperv.h
> >> @@ -396,8 +396,10 @@ static inline int hv_deposit_memory(u64 partition_id, u64 status)
> >>
> >>   #if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
> >>   u8 __init get_vtl(void);
> >> +void mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0);
> >>   #else
> >>   static inline u8 get_vtl(void) { return 0; }
> >> +static inline void mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0) {}
> >
> > Is this stub needed? Maybe I missed something, but it looks to me like none
> > of the code that calls this gets built unless CONFIG_HYPERV_VTL_MODE is set.
> > See further comments about stubs in Patch 8 of this series.
> >
> 
> Config dependencies would handle such cases, and this is not required. I
> saw similar stubs added in the code, so I thought this is a norm that
> should be followed, and not rely on config dependencies.
> I can remove it.
> 

Others might disagree with me, but I don't think it's the norm to add
stubs when they aren't truly needed. As you can see from some of my
other comments, I look for ways to eliminate stubs. Stubs are indicative
of a boundary between separately built components, and I generally
try to minimize the surface area of such boundaries. A large surface area
often means that the overall design could be improved by re-thinking
which code goes with which component.

Michael