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

[Roman Kisel <romank@linux.microsoft.com>]

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@linux.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.

> 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).

That said, I think the logic is solid. Appreciate your question and your 
help! Will be glad to discuss other concerns should you have any.

> 
> - Saurabh

-- 
Thank you,
Roman