Re: [PATCH v4 2/2] coco: guest: arm64: Drop dummy RSI platform device stub

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:

> Greg KH <gregkh@linuxfoundation.org> writes:
>
>> On Wed, May 13, 2026 at 12:28:12PM +0530, Aneesh Kumar K.V wrote:
>>> Catalin Marinas <catalin.marinas@arm.com> writes:
>>> 
>>> > + Suzuki again
>>> >
>>> > On Mon, Apr 27, 2026 at 11:46:15AM +0530, Aneesh Kumar K.V (Arm) wrote:
>>> >> The SMCCC firmware driver now creates the `arm-smccc` platform device
>>> >> and also creates the CCA auxiliary devices once the RSI ABI is
>>> >> discovered. This makes the arch-specific arm64_create_dummy_rsi_dev()
>>> >> helper redundant. Remove the arm-cca-dev platform device registration
>>> >> and let the SMCCC probe manage the RSI device.
>>> >> 
>>> >> systemd match on platform:arm-cca-dev for confidential vm detection [1].
>>> >> Losing the platform device registration can break that. Keeping this
>>> >> removal in its own change makes it easy to revert if that regression
>>> >> blocks the rollout.
>>> >> 
>>> >> [1] https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com
>>> >
>>> > I wouldn't merge this now given that systemd checks this file. Could we
>>> > have a symbolic link instead for some time until systemd eventually gets
>>> > updated (years?).
>>> >
>>> 
>>> I’ll add this in the next revision.
>>> 
>>> static int create_rsi_compat_link(struct device *target_dev)
>>> {
>>> 	struct kobject *platform_kobj;
>>> 	/*
>>> 	 * target_dev is:
>>> 	 * /sys/devices/platform/arm-smccc/arm_cca_guest.arm-rsi-dev.0
>>> 	 * Create compat link /sys/devices/platform/arm-cca-dev
>>> 	 */
>>> 	platform_kobj = target_dev->kobj.parent->parent;
>>
>> What?  That is crazy, you don't know that is always going to be ok.
>>
>>> 	return sysfs_create_link(platform_kobj,
>>> 				 &target_dev->kobj,
>>> 				 "arm-cca-dev");
>>
>> No, don't do that, if a driver calls a sysfs* function, something is
>> almost always wrong.  Don't be making random sysfs symlinks please.
>>
>
> Sure, but could you explain why this is wrong? Below is the full version
> of the updated patch.
>
> coco: guest: arm64 Replace RSI platform device with compat symlink
>
> The SMCCC firmware driver now creates the arm-smccc platform device and
> registers the RSI device as an auxiliary device once the RSI ABI has been
> discovered. This makes the arch-specific arm64 arm-cca-dev platform device
> redundant.
>
> Remove the arm64 platform device stub and let the SMCCC core manage RSI
> device creation.
>
> This changes the real device location from the old platform device path to:
>
>   /sys/devices/platform/arm-smccc/arm_cca_guest.arm-rsi-dev.0
>
> Keep userspace compatibility by creating a sysfs symlink at the old path:
>
>   /sys/devices/platform/arm-cca-dev
>
> A Debian Code Search check found systemd matching on the old
> platform:arm-cca-dev device path for confidential VM detection. No other
> userspace dependency on the old platform device path was found, but keeping
> the compatibility symlink avoids breaking existing systemd-based detection
> [1].
>
> [1] https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com
>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>
> modified   arch/arm64/kernel/rsi.c
> @@ -159,18 +159,3 @@ void __init arm64_rsi_init(void)
>  
>  	static_branch_enable(&rsi_present);
>  }
> -
> -static struct platform_device rsi_dev = {
> -	.name = "arm-cca-dev",
> -	.id = PLATFORM_DEVID_NONE
> -};
> -
> -static int __init arm64_create_dummy_rsi_dev(void)
> -{
> -	if (is_realm_world() &&
> -	    platform_device_register(&rsi_dev))
> -		pr_err("failed to register rsi platform device\n");
> -	return 0;
> -}
> -
> -arch_initcall(arm64_create_dummy_rsi_dev)
> modified   drivers/firmware/smccc/rmm.c
> @@ -4,13 +4,31 @@
>   */
>  
>  #include <linux/auxiliary_bus.h>
> +#include <linux/sysfs.h>
> +#include <linux/device.h>
>  
>  #include "rmm.h"
>  
> +static int create_rsi_compat_link(struct device *target_dev)
> +{
> +	struct kobject *platform_kobj;
> +	/*
> +	 * target_dev is:
> +	 * /sys/devices/platform/arm-smccc/arm_cca_guest.arm-rsi-dev.0
> +	 * Create compat link /sys/devices/platform/arm-cca-dev
> +	 */
> +	platform_kobj = target_dev->kobj.parent->parent;
> +
> +	return sysfs_create_link(platform_kobj,
> +				 &target_dev->kobj,
> +				 "arm-cca-dev");
> +}
> +
>  void __init register_rsi_device(struct platform_device *pdev)
>  {
>  	unsigned long ret;
>  	unsigned long ver_lower, ver_higher;
> +	struct auxiliary_device *adev;
>  
>  	if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_SMC)
>  		return;
> @@ -19,6 +37,10 @@ void __init register_rsi_device(struct platform_device *pdev)
>  	if (ret != RSI_SUCCESS)
>  		return;
>  
> -	__devm_auxiliary_device_create(&pdev->dev,
> -				       "arm_cca_guest", RSI_DEV_NAME, NULL, 0);
> +	adev = __devm_auxiliary_device_create(&pdev->dev,
> +				"arm_cca_guest", RSI_DEV_NAME, NULL, 0);
> +	if (!adev)
> +		return;
> +
> +	create_rsi_compat_link(&adev->dev);
>  }

To make this clear, the above implies that adev parent is a platform
device and hence target_dev->kobj.parent->parent should be platform
kobject?

>
>
>>
>> If userspace can not find the device anymore, that's fine, that's how
>> sysfs works, devices move around all the time.  Especially platform
>> devices as those are almost always not supposed to be platform devices :)
>>
>> thanks,
>>
>> greg k-h
>

-aneesh