RE: [PATCH hyperv-next v4 1/6] arm64: hyperv: Use SMCCC to detect hypervisor presence

[Michael Kelley]

From: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, February 11, 2025 5:43 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 +++++++++++++++++++++++++++----
>  arch/arm64/include/asm/mshyperv.h |  5 ++++
>  2 files changed, 43 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index fc49949b7df6..fe6185bf3bf2 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -27,6 +27,36 @@ int hv_get_hypervisor_version(union
> hv_hypervisor_version_info *info)
>  	return 0;
>  }
> 
> +static bool hyperv_detect_via_acpi(void)
> +{
> +	if (acpi_disabled)
> +		return false;
> +#if IS_ENABLED(CONFIG_ACPI)
> +	/* Hypervisor ID is only available in ACPI v6+. */

The comment is correct, but to me doesn't tell the full story.
I initially wondered why the revision test < 6 was being done,
since Hyper-V for ARM64 always provides ACPI 6.x or greater.
But the test is 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, so just return false instead of testing
a bogus hypervisor_id field. Maybe a comment would help
explain it to someone like me who was confused. :-)

> +	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 hyperv_detect_via_smccc(void)
> +{
> +	struct arm_smccc_res res = {};
> +
> +	if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
> +		return false;
> +	arm_smccc_1_1_hvc(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
> +	if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
> +		return false;
> +
> +	return res.a0 == ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_0 &&
> +		res.a1 == ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_1 &&
> +		res.a2 == ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_2 &&
> +		res.a3 == ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_3;
> +}
> +
>  static int __init hyperv_init(void)
>  {
>  	struct hv_get_vp_registers_output	result;
> @@ -35,13 +65,11 @@ 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 */
> @@ -72,6 +100,11 @@ static int __init hyperv_init(void)
>  		return ret;
>  	}
> 
> +	ms_hyperv.vtl = get_vtl();

The above statement looks like it will fail to compile on
arm64 since the get_vtl() function is entirely on the x86
side until Patch 3 of this series. As of this Patch 1, there's
no declaration of get_vtl() available to arm64.

> +	/* Report if non-default VTL */
> +	if (ms_hyperv.vtl > 0)
> +		pr_info("Linux runs in Hyper-V Virtual Trust Level\n");

Could this message include the VTL number as well? In the long
run, I think there will be code at non-zero VTLs other than VTL 2.

> +
>  	ms_hyperv_late_init();
> 
>  	hyperv_initialized = true;
> diff --git a/arch/arm64/include/asm/mshyperv.h
> b/arch/arm64/include/asm/mshyperv.h
> index 2e2f83bafcfb..a6d7eb9e167b 100644
> --- a/arch/arm64/include/asm/mshyperv.h
> +++ b/arch/arm64/include/asm/mshyperv.h
> @@ -50,4 +50,9 @@ static inline u64 hv_get_msr(unsigned int reg)
> 
>  #include <asm-generic/mshyperv.h>
> 
> +#define ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_0	0x4d32ba58U
> +#define ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_1	0xcd244764U
> +#define ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_2	0x8eef6c75U
> +#define ARM_SMCCC_VENDOR_HYP_UID_HYPERV_REG_3	0x16597024U
> +
>  #endif
> --
> 2.43.0
>