RE: [PATCH hyperv-next v5 02/11] arm64: hyperv: Use SMCCC to detect hypervisor presence

[Michael Kelley]

From: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> Sent: Friday, March 7, 2025 2:03 PM
> 
> The arm64 Hyper-V startup path relies on ACPI to detect
> running under a Hyper-V compatible hypervisor. That
> doesn't work on non-ACPI systems.
> 
> Hoist the ACPI detection logic into a separate function. Then
> use the vendor-specific hypervisor service call (implemented
> recently in Hyper-V) via SMCCC in the non-ACPI case.
> 
> Signed-off-by: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/hyperv/mshyperv.c | 43 +++++++++++++++++++++++++++++++-----
>  1 file changed, 38 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index 29fcfd595f48..c647db56fd6b 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -27,6 +27,41 @@ int hv_get_hypervisor_version(union hv_hypervisor_version_info
> *info)
>  	return 0;
>  }
> 
> +static bool __init hyperv_detect_via_acpi(void)
> +{
> +	if (acpi_disabled)
> +		return false;
> +#if IS_ENABLED(CONFIG_ACPI)
> +	/*
> +	 * Hypervisor ID is only available in ACPI v6+, and the
> +	 * structure layout was extended in v6 to accommodate that
> +	 * new field.
> +	 *
> +	 * At the very minimum, this check makes sure not to read
> +	 * past the FADT structure.
> +	 *
> +	 * It is also needed to catch running in some unknown
> +	 * non-Hyper-V environment that has ACPI 5.x or less.
> +	 * In such a case, it can't be Hyper-V.
> +	 */
> +	if (acpi_gbl_FADT.header.revision < 6)
> +		return false;
> +	return strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8) == 0;
> +#else
> +	return false;
> +#endif
> +}
> +
> +static bool __init hyperv_detect_via_smccc(void)
> +{
> +	uuid_t hyperv_uuid = UUID_INIT(
> +		0x4d32ba58, 0x4764, 0xcd24,
> +		0x75, 0x6c, 0xef, 0x8e,
> +		0x24, 0x70, 0x59, 0x16);
> +
> +	return arm_smccc_hyp_present(&hyperv_uuid);
> +}
> +
>  static int __init hyperv_init(void)
>  {
>  	struct hv_get_vp_registers_output	result;
> @@ -35,13 +70,11 @@ static int __init hyperv_init(void)
> 
>  	/*
>  	 * Allow for a kernel built with CONFIG_HYPERV to be running in
> -	 * a non-Hyper-V environment, including on DT instead of ACPI.
> +	 * a non-Hyper-V environment.
> +	 *
>  	 * In such cases, do nothing and return success.
>  	 */
> -	if (acpi_disabled)
> -		return 0;
> -
> -	if (strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8))
> +	if (!hyperv_detect_via_acpi() && !hyperv_detect_via_smccc())
>  		return 0;
> 
>  	/* Setup the guest ID */
> --
> 2.43.0
> 

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>