linux-kernel - Re: [PATCH hyperv-next v6 02/11] arm64: hyperv: Use SMCCC to detect hypervisor presence

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Message-ID: <42a1a90b-ae3e-47df-ad6c-500055e69218@linux.microsoft.com>
Date: Thu, 20 Mar 2025 11:47:18 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>, Mark Rutland <mark.rutland@....com>
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Subject: Re: [PATCH hyperv-next v6 02/11] arm64: hyperv: Use SMCCC to detect
 hypervisor presence



On 3/19/2025 3:11 PM, Michael Kelley wrote:
> From: Mark Rutland <mark.rutland@....com> Sent: Monday, March 17, 2025 4:38 AM

>>
>> The 'acpi_disabled' variable doesn't exist for !CONFIG_ACPI, so its use
>> prior to the ifdeffery looks misplaced.
> 
> FWIW, include/linux/acpi.h has
> 
> #define acpi_disabled 1
> 
> when !CONFIG_ACPI.  But I agree that using a stub is better.

Thanks, Michael! I didn't "fact-checked" what was suggested indeed.
Agreed that the stub makes the code look better.

> 
> Michael
> 
>>
>> Usual codestyle is to avoid ifdeffery if possible, using IS_ENABLED().
>> Otherwise, use a stub, e.g.
>>
>> | #ifdef CONFIG_ACPI
>> | static bool __init hyperv_detect_via_acpi(void)
>> | {
>> | 	if (acpi_disabled)
>> | 		return false;
>> |
>> | 	if (acpi_gbl_FADT.header.revision < 6)
>> | 		return false;
>> |
>> | 	return strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8) == 0;
>> | }
>> | #else
>> | static inline bool hyperv_detect_via_acpi(void) { return false; }
>> | #endif
>>
>> Mark.
>>
>>> +static bool __init hyperv_detect_via_smccc(void)
>>> +{
>>> +	uuid_t hyperv_uuid = UUID_INIT(
>>> +		0x4d32ba58, 0x4764, 0xcd24,
>>> +		0x75, 0x6c, 0xef, 0x8e,
>>> +		0x24, 0x70, 0x59, 0x16);
>>> +
>>> +	return arm_smccc_hyp_present(&hyperv_uuid);
>>> +}
>>> +
>>>   static int __init hyperv_init(void)
>>>   {
>>>   	struct hv_get_vp_registers_output	result;
>>> @@ -35,13 +70,11 @@ 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 */
>>> --
>>> 2.43.0
>>>
> 

-- 
Thank you,
Roman