RE: [PATCH hyperv-next v4 1/6] arm64: hyperv: Use SMCCC to detect hypervisor presence
From: Michael Kelley <hidden>
Date: 2025-02-19 23:13:30
Also in:
linux-arch, linux-devicetree, linux-hyperv, linux-pci, lkml
From: Roman Kisel <redacted> Sent: Tuesday, February 11, 2025 5:43 PM
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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 <redacted> --- 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(unionhv_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. :-)
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+ 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.
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+ ms_hyperv_late_init(); hyperv_initialized = true;diff --git a/arch/arm64/include/asm/mshyperv.hb/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