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

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

On 4/27/2026 11:08 AM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Thursday, April 23, 2026 5:42 AM
>>
>> Add the arm64 variant of mshv_vtl_return_call() to support the MSHV_VTL
>> driver on arm64. This function enables the transition between Virtual
>> Trust Levels (VTLs) in MSHV_VTL when the kernel acts as a paravisor.
>>
>> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
>> Reviewed-by: Roman Kisel <vdso@mailbox.org>
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
>> ---
>>   arch/arm64/hyperv/Makefile        |   1 +
>>   arch/arm64/hyperv/hv_vtl.c        | 158 ++++++++++++++++++++++++++++++
>>   arch/arm64/include/asm/mshyperv.h |  13 +++
>>   arch/x86/include/asm/mshyperv.h   |   2 -
>>   drivers/hv/mshv_vtl.h             |   3 +
>>   include/asm-generic/mshyperv.h    |   2 +
>>   6 files changed, 177 insertions(+), 2 deletions(-)
>>   create mode 100644 arch/arm64/hyperv/hv_vtl.c
>>
> 
> [snip]
> 
>> diff --git a/arch/arm64/include/asm/mshyperv.h b/arch/arm64/include/asm/mshyperv.h
>> index 585b23a26f1b..9eb0e5999f29 100644
>> --- a/arch/arm64/include/asm/mshyperv.h
>> +++ b/arch/arm64/include/asm/mshyperv.h
>> @@ -60,6 +60,18 @@ static inline u64 hv_get_non_nested_msr(unsigned int reg)
>>   				ARM_SMCCC_SMC_64,		\
>>   				ARM_SMCCC_OWNER_VENDOR_HYP,	\
>>   				HV_SMCCC_FUNC_NUMBER)
>> +
>> +struct mshv_vtl_cpu_context {
>> +/*
>> + * x18 is managed by the hypervisor. It won't be reloaded from this array.
>> + * It is included here for convenience in array indexing.
>> + * 'rsvd' field serves as alignment padding so q[] starts at offset 32*8=256.
>> + */
>> +	__u64 x[31];
>> +	__u64 rsvd;
>> +	__uint128_t q[32];
>> +};
>> +
>>   #ifdef CONFIG_HYPERV_VTL_MODE
>>   /*
>>    * Get/Set the register. If the function returns `1`, that must be done via
>> @@ -69,6 +81,7 @@ static inline int hv_vtl_get_set_reg(struct hv_register_assoc *regs,
>> bool set, b
>>   {
>>   	return 1;
>>   }
>> +
> 
> This appears to be a spurious blank line being added since there
> are no other changes in the vicinity.

Acked.

> 
>>   #endif
>>
>>   #include <asm-generic/mshyperv.h>
>> 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.

Regards,
Naman