Re: [PATCH hyperv-next v6 02/11] arm64: hyperv: Use SMCCC to detect hypervisor presence

Mark Rutland
2025-03-17T11:38:08Z

On Fri, Mar 14, 2025 at 05:19:22PM -0700, Roman Kisel wrote:
> 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@linux.microsoft.com>
> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> ---
> 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 2265ea5ce5ad..c5b03d3af7c5 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
> +}
> +

The 'acpi_disabled' variable doesn't exist for !CONFIG_ACPI, so its use
prior to the ifdeffery looks misplaced.

Usual codestyle is to avoid ifdeffery if possible, using IS_ENABLED().
Otherwise, use a stub, e.g.

| #ifdef CONFIG_ACPI
| static bool __init hyperv_detect_via_acpi(void)
| {
| if (acpi_disabled)
| return false;
|
| if (acpi_gbl_FADT.header.revision < 6)
| return false;
|
| return strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8) == 0;
| }
| #else
| static inline bool hyperv_detect_via_acpi(void) { return false; }
| #endif

Mark.

> +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
>