Re: [PATCH v5 2/6] firmware: smccc: Detect hypervisor via RSI host call in CCA Realms

[Kameron Carr <kameroncarr@linux.microsoft.com>]

On 10/29/2026 3:54 PM, Catalin Marinas wrote:
> On Fri, Oct 02, 2026 at 02:57:21PM -0700, Kameron Carr wrote:
>> +#ifdef CONFIG_ARM64
>> +static struct rsi_host_call uuid_hc;
>> +static DEFINE_SPINLOCK(uuid_hc_lock);
>> +
>> +/*
>> + * Helper function to get the hypervisor UUID via an RsiHostCall.
>> + */
>> +static void arm_smccc_realm_get_hypervisor_uuid(struct arm_smccc_res *res)
>> +{
>> +	guard(spinlock_irqsave)(&uuid_hc_lock);
>> +
>> +	memset(&uuid_hc, 0, sizeof(uuid_hc));
>> +	uuid_hc.gprs[0] = ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID;
>> +
>> +	if (rsi_host_call(__pa_symbol(&uuid_hc)) != RSI_SUCCESS) {
>> +		res->a0 = SMCCC_RET_NOT_SUPPORTED;
>> +		return;
>> +	}
>> +
>> +	res->a0 = uuid_hc.gprs[0];
>> +	res->a1 = uuid_hc.gprs[1];
>> +	res->a2 = uuid_hc.gprs[2];
>> +	res->a3 = uuid_hc.gprs[3];
>> +}
>> +#endif
>> +
>>  bool arm_smccc_hypervisor_has_uuid(const uuid_t *hyp_uuid)
>>  {
>>  	struct arm_smccc_res res = {};
>>  	uuid_t uuid;
>>  
>> -	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
>> +#ifdef CONFIG_ARM64
>> +	if (is_realm_world())
>> +		arm_smccc_realm_get_hypervisor_uuid(&res);
>> +	else
>> +#endif
>> +		arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID,
>> +				     &res);
> 
> Is this meant to be generic? I'm not sure the trick to pass
> ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID in gprs[0] and forward it to the
> host is a documented ABI. I think it even confuses other realm guests
> that don't expect this (e.g. kvm_init_hyp_services()).
> 
> I thought realms should stick to the RSI interface and not try to guess
> the host hypervisor. But if you do need this, a better fit would be in
> hyperv_detect_via_smccc() (if realms don't boot with ACPI).
> 
> I'll leave it to Sudeep or Suzuki to comment further.

I moved this code to a Hyper-V specific location as you suggested.

Let me know if you have any other concerns.

[v6] https://lore.kernel.org/all/20261031001412.4150488-1-kameroncarr@linux.microsoft.com/

-- 
Regards,
Kameron