RE: [PATCH 2/4] Drivers: hv: Use nested hypercall for post message and signal event

[Michael Kelley]

From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, June 10, 2025 4:52 PM
> 
> When running nested, these hypercalls must be sent to the L0 hypervisor
> or vmbus will fail.

s/vmbus/VMBus/

> 
> Add ARM64 stubs for the nested hypercall helpers to not break
> compilation (nested is still only supported in x86).
> 
> Signed-off-by: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/mshyperv.h | 10 ++++++++++
>  drivers/hv/connection.c           |  3 +++
>  drivers/hv/hv.c                   |  3 +++
>  3 files changed, 16 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/mshyperv.h
> b/arch/arm64/include/asm/mshyperv.h
> index b721d3134ab6..893d6a2e8dab 100644
> --- a/arch/arm64/include/asm/mshyperv.h
> +++ b/arch/arm64/include/asm/mshyperv.h
> @@ -53,6 +53,16 @@ static inline u64 hv_get_non_nested_msr(unsigned int reg)
>  	return hv_get_msr(reg);
>  }
> 
> +static inline u64 hv_do_nested_hypercall(u64 control, void *input, void *output)
> +{
> +	return U64_MAX;
> +}
> +
> +static inline u64 hv_do_fast_nested_hypercall8(u64 control, u64 input1)
> +{
> +	return U64_MAX;
> +}

I think the definitions of hv_do_nested_hypercall() and
hv_do_fast_nested_hypercall8() are architecture independent. All
they do is add the HV_HYPERCALL_NESTED flag, which when
implemented for ARM64, will presumably be the same flag as
currently defined for x86.  As such, couldn't the definitions of
hv_do_nested_hypercall() and hv_do_fast_nested_hypercall8()
be moved to asm-generic/mshyperv.h? Then stubs would not
be needed for ARM64. These two functions would never be
called on ARM64 because hv_nested is never true on ARM64
(at least for now), but the code would compile. And if either
function was erroneously called on ARM64, presumably
Hyper-V would return an error because HV_HYPERCALL_NESTED
is set.

> +
>  /* SMCCC hypercall parameters */
>  #define HV_SMCCC_FUNC_NUMBER	1
>  #define HV_FUNC_ID	ARM_SMCCC_CALL_VAL(			\
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index be490c598785..992022bc770c 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -518,6 +518,9 @@ void vmbus_set_event(struct vmbus_channel *channel)
>  					 channel->sig_event, 0);
>  		else
>  			WARN_ON_ONCE(1);
> +	} else if (hv_nested) {
> +		hv_do_fast_nested_hypercall8(HVCALL_SIGNAL_EVENT,
> +					     channel->sig_event);
>  	} else {
>  		hv_do_fast_hypercall8(HVCALL_SIGNAL_EVENT, channel->sig_event);
>  	}
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 308c8f279df8..99b73e779bf0 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -84,6 +84,9 @@ int hv_post_message(union hv_connection_id connection_id,
>  						   sizeof(*aligned_msg));
>  		else
>  			status = HV_STATUS_INVALID_PARAMETER;
> +	} else if (hv_nested) {
> +		status = hv_do_nested_hypercall(HVCALL_POST_MESSAGE,
> +						aligned_msg, NULL);
>  	} else {
>  		status = hv_do_hypercall(HVCALL_POST_MESSAGE,
>  					 aligned_msg, NULL);

Are HVCALL_SIGNAL_EVENT and HVCALL_POST_MESSAGE the only two
hypercalls that are ever expected to need a "nested" version? I'm
wondering if the function hv_do_nested_hypercall() and
hv_do_fast_nested_hypercall8() could be dropped entirely, and just
pass the first argument to hv_do_hypercall() or hv_do_fast_hypercall8()
as <hypercall_name> | HV_HYPERCALL_NESTED. For only two cases, a
little bit of open coding might be preferable to the overhead of defining
functions just to wrap the or'ing of HV_HYPERCALL_NESTED. 

The code above could then look like:

	} else {
		u64 control = HVCALL_POST_MESSAGE;

		control |= hv_nested ? HV_HYPERCALL_NESTED : 0;
		status = hv_do_hypercall(control, aligned_msg, NULL);
	}

Again, ARM64 is implicitly handled because hv_nested is never set.

This is just a suggestion. It's motivated by the fact that we already have
three flavors of hypercall for HVCALL_SIGNAL_EVENT and
HVCALL_POST_MESSAGE, and I was looking for a way to avoid adding
a fourth flavor. But it's a marginal win, and if you prefer to keep the
inline functions, I'm OK with that.

Michael