On Mon, Aug 05, 2024 at 08:17:05AM -0700, Roman Kisel wrote:

On 8/4/2024 8:53 PM, Saurabh Singh Sengar wrote:

quoted

On Fri, Jul 26, 2024 at 03:59:04PM -0700, Roman Kisel wrote:

quoted

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 <redacted>
---
 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.

I can add a comment about that, thought that would look as too much
detail to refer to a version of the Windows insiders build in the
comments in this code. Another option would be to entrench the logic
in if statements which felt gross as there is a fallback.

I'll leave the decision to your judgment.

quoted

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 ?

The function won't panic if that's what you're asking about, i.e.
safe for runtime. That won't break the build either as it relies on
the SMCCC spec, and that uses the smc or hvc instructions (the code
does expect hvc to be the conduit and checks for that being the
case). The hypervisor doesn't inject the exception in the guest for
the unknown call, just returns SMCCC_RET_NOT_SUPPORTED in the first
output register (the hypervisor got a unit-test for that, too).

Looks good, have you considered checking for SMCCC_RET_NOT_SUPPORTED ?

- Saurabh