linux-kernel - Re: [PATCH v3 1/7] arm64: hyperv: Use SMC to detect hypervisor presence

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Message-ID: <20240805035340.GA13276@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Sun, 4 Aug 2024 20:53:40 -0700
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Roman Kisel <romank@...ux.microsoft.com>
Cc: arnd@...db.de, bhelgaas@...gle.com, bp@...en8.de,
	catalin.marinas@....com, dave.hansen@...ux.intel.com,
	decui@...rosoft.com, haiyangz@...rosoft.com, hpa@...or.com,
	kw@...ux.com, kys@...rosoft.com, lenb@...nel.org,
	lpieralisi@...nel.org, mingo@...hat.com, rafael@...nel.org,
	robh@...nel.org, tglx@...utronix.de, wei.liu@...nel.org,
	will@...nel.org, linux-acpi@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, x86@...nel.org, apais@...rosoft.com,
	benhill@...rosoft.com, ssengar@...rosoft.com,
	sunilmut@...rosoft.com, vdso@...bites.dev
Subject: Re: [PATCH v3 1/7] arm64: hyperv: Use SMC to detect hypervisor
 presence

On Fri, Jul 26, 2024 at 03:59:04PM -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,
> use the new SMC added recently to Hyper-V to use in the
> non-ACPI case.
> 
> Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
> ---
>  arch/arm64/hyperv/mshyperv.c      | 36 ++++++++++++++++++++++++++-----
>  arch/arm64/include/asm/mshyperv.h |  5 +++++
>  2 files changed, 36 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index b1a4de4eee29..341f98312667 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -27,6 +27,34 @@ 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+. */
> +	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_smc(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);
> +
> +	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;
> +}

As you mentioned in the cover letter this is supported in latest Hyper-V hypervisor,
can we add a comment about it, specifying the exact version in it would be great.

If someone attempts to build non-ACPI kernel on older Hyper-V what is the
behaviour of this function, do we need to safeguard or handle that case ?

- Saurabh