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

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Message-ID: <Z9gJlQgV3hm1kxY0@J2N7QTR9R3.cambridge.arm.com>
Date: Mon, 17 Mar 2025 11:37:57 +0000
From: Mark Rutland <mark.rutland@....com>
To: Roman Kisel <romank@...ux.microsoft.com>
Cc: arnd@...db.de, bhelgaas@...gle.com, bp@...en8.de,
	catalin.marinas@....com, conor+dt@...nel.org,
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	sunilmut@...rosoft.com
Subject: Re: [PATCH hyperv-next v6 02/11] arm64: hyperv: Use SMCCC to detect
 hypervisor presence

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@...ux.microsoft.com>
> Reviewed-by: Michael Kelley <mhklinux@...look.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
>