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Message-ID:
<SN6PR02MB4157DB2F52501309D52D0870D4C52@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 19 Feb 2025 23:13:28 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, "arnd@...db.de" <arnd@...db.de>,
"bhelgaas@...gle.com" <bhelgaas@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
"catalin.marinas@....com" <catalin.marinas@....com>, "conor+dt@...nel.org"
<conor+dt@...nel.org>, "dave.hansen@...ux.intel.com"
<dave.hansen@...ux.intel.com>, "decui@...rosoft.com" <decui@...rosoft.com>,
"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, "hpa@...or.com"
<hpa@...or.com>, "krzk+dt@...nel.org" <krzk+dt@...nel.org>, "kw@...ux.com"
<kw@...ux.com>, "kys@...rosoft.com" <kys@...rosoft.com>,
"lpieralisi@...nel.org" <lpieralisi@...nel.org>,
"manivannan.sadhasivam@...aro.org" <manivannan.sadhasivam@...aro.org>,
"mingo@...hat.com" <mingo@...hat.com>, "robh@...nel.org" <robh@...nel.org>,
"ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
"tglx@...utronix.de" <tglx@...utronix.de>, "wei.liu@...nel.org"
<wei.liu@...nel.org>, "will@...nel.org" <will@...nel.org>,
"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>, "linux-hyperv@...r.kernel.org"
<linux-hyperv@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "linux-pci@...r.kernel.org"
<linux-pci@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>
CC: "benhill@...rosoft.com" <benhill@...rosoft.com>, "bperkins@...rosoft.com"
<bperkins@...rosoft.com>, "sunilmut@...rosoft.com" <sunilmut@...rosoft.com>
Subject: RE: [PATCH hyperv-next v4 1/6] arm64: hyperv: Use SMCCC to detect
hypervisor presence
From: Roman Kisel <romank@...ux.microsoft.com> 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@...ux.microsoft.com>
> ---
> 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
>
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