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

[Catalin Marinas <catalin.marinas@arm.com>]

+ Suzuki

On Fri, Oct 02, 2026 at 02:57:21PM -0700, Kameron Carr wrote:
> Modify arm_smccc_hypervisor_has_uuid() to check is_realm_world() and
> use rsi_host_call() to query the hypervisor vendor UUID when inside a
> Realm. The realm path is factored into a helper,
> arm_smccc_realm_get_hypervisor_uuid(), that owns a file-static
> rsi_host_call buffer (uuid_hc) serialized by a spinlock.
> 
> The RSI-specific includes, file-static state and helper are guarded with
> CONFIG_ARM64 because <linux/arm-rsi-cmds.h> does not build on 32-bit
> ARM.
> 
> For non-Realm environments, the existing arm_smccc_1_1_invoke() path
> is unchanged.
> 
> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
> ---
>  drivers/firmware/smccc/smccc.c | 41 +++++++++++++++++++++++++++++++++-
>  1 file changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/smccc/smccc.c b/drivers/firmware/smccc/smccc.c
> index afc2e4d..7e77170 100644
> --- a/drivers/firmware/smccc/smccc.c
> +++ b/drivers/firmware/smccc/smccc.c
> @@ -14,6 +14,12 @@
>  
>  #include <asm/archrandom.h>
>  
> +#ifdef CONFIG_ARM64
> +#include <linux/arm-rsi-cmds.h>
> +#include <linux/cleanup.h>
> +#include <linux/spinlock.h>
> +#endif
> +
>  static u32 smccc_version = ARM_SMCCC_VERSION_1_0;
>  static enum arm_smccc_conduit smccc_conduit = SMCCC_CONDUIT_NONE;
>  
> @@ -69,12 +75,45 @@ s32 arm_smccc_get_soc_id_revision(void)
>  }
>  EXPORT_SYMBOL_GPL(arm_smccc_get_soc_id_revision);
>  
> +#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.

-- 
Catalin