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

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Message-ID: <20240805154632.GA11961@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Mon, 5 Aug 2024 08:46:32 -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 Mon, Aug 05, 2024 at 08:17:05AM -0700, Roman Kisel wrote:
> 
> 
> On 8/4/2024 8:53 PM, Saurabh Singh Sengar wrote:
> >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.
> >
> 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.

> 
> >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