Re: [PATCH v19 2/7] firmware: arm_rmm: Check for RMI support at init

[Gavin Shan <gshan@redhat.com>]

On 9/24/26 11:51 PM, Suzuki K Poulose wrote:
> From: Steven Price <steven.price@arm.com>
> 
> Query the RMI version number and check if it is a compatible version.
> The first two feature registers are read and exposed for future code to
> use.
> 
> We only support this for Little Endian kernels, the Big Endian kernel
> support is anyway marked BROKEN and is being removed.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> v19:
>   * Read all implemented RmiFeatureRegisters - 5
>   * Use ARRAY_SIZE(rmi_feat_reg_cache) for the loop in rmi_read_features()
>   * Fold rmi_features() into rmi_read_features
>   * Fix comment for rmi_smccc_invoke()
>   * Drop default y
>   * Add retry for RMI_BLOCKED and return to caller
> v18:
>   * Always use arm_smccc_1_2_invoke() for all RMIs making sure the unsused
>     parameters are 0 - Sashiko
>   * Move rmi_features() calls away from the arm-rmi-cmds.h to rmi.c - Gavin
> v17:
>   * Rename ARM_RMM to ARM_RMM_RMI to make it easier to add Guest facing RSI
>     support, which is also in progress
> v16:
>   * Update Kconfig text to include PCIe TDISP.
>   * Export rmi_feat_reg() here rather than in a later commit.
> v15:
>   * The code is moved again, this time into the 'firmware' directory.
> v14:
>   * This moves the basic RMI setup into the 'kernel' directory. This is
>     because RMI will be used for some features outside of KVM so should
>     be available even if KVM isn't compiled in.
> ---
>   arch/arm64/Kconfig                |   1 +
>   arch/arm64/kernel/cpufeature.c    |   1 +
>   drivers/firmware/Kconfig          |   1 +
>   drivers/firmware/Makefile         |   1 +
>   drivers/firmware/arm_rmm/Kconfig  |  25 +++++++
>   drivers/firmware/arm_rmm/Makefile |   2 +
>   drivers/firmware/arm_rmm/rmi.c    | 109 ++++++++++++++++++++++++++++++
>   include/linux/arm-rmi-cmds.h      |  48 +++++++++++++
>   8 files changed, 188 insertions(+)
>   create mode 100644 drivers/firmware/arm_rmm/Kconfig
>   create mode 100644 drivers/firmware/arm_rmm/Makefile
>   create mode 100644 drivers/firmware/arm_rmm/rmi.c
>   create mode 100644 include/linux/arm-rmi-cmds.h
> 

Some nitpicks below, with them addressed:

Reviewed-by: Gavin Shan <gshan@redhat.com>

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b5a51b0ef9440..ff9565d3ffa59 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -38,6 +38,7 @@ config ARM64
>   	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>   	select ARCH_HAS_MEM_ENCRYPT
>   	select ARCH_SUPPORTS_MSEAL_SYSTEM_MAPPINGS
> +	select ARCH_SUPPORTS_RMM
>   	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
>   	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>   	select ARCH_HAS_NONLEAF_PMD_YOUNG if ARM64_HAFT
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 32102c3912fa7..e8b29983b0021 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -293,6 +293,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar3[] = {
>   static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_CSV3_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_CSV2_SHIFT, 4, 0),
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_RME_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_DIT_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_AMU_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_EL1_MPAM_SHIFT, 4, 0),
> diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
> index b7cc11e4fbfa6..62660bf520a8d 100644
> --- a/drivers/firmware/Kconfig
> +++ b/drivers/firmware/Kconfig
> @@ -310,5 +310,6 @@ source "drivers/firmware/samsung/Kconfig"
>   source "drivers/firmware/smccc/Kconfig"
>   source "drivers/firmware/tegra/Kconfig"
>   source "drivers/firmware/xilinx/Kconfig"
> +source "drivers/firmware/arm_rmm/Kconfig"
>   
>   endmenu
> diff --git a/drivers/firmware/Makefile b/drivers/firmware/Makefile
> index be46f1e1dc77f..196a650ccf025 100644
> --- a/drivers/firmware/Makefile
> +++ b/drivers/firmware/Makefile
> @@ -39,3 +39,4 @@ obj-y				+= samsung/
>   obj-y				+= smccc/
>   obj-y				+= tegra/
>   obj-y				+= xilinx/
> +obj-y				+= arm_rmm/
> diff --git a/drivers/firmware/arm_rmm/Kconfig b/drivers/firmware/arm_rmm/Kconfig
> new file mode 100644
> index 0000000000000..a37cba6647360
> --- /dev/null
> +++ b/drivers/firmware/arm_rmm/Kconfig
> @@ -0,0 +1,25 @@
> +
> +config ARCH_SUPPORTS_RMM
> +	bool
> +
> +config ARM_RMM_RMI
> +	bool "Realm Management Interface (RMI) Support"
> +	depends on ARCH_SUPPORTS_RMM
> +	help
> +	  Support the Realm Management Monitor (RMM) on Arm systems that
> +	  implement the Realm Management Extension (RME), as defined by the
> +	  Arm Confidential Compute Architecture.
> +
> +	  The RMM runs at EL2 in the Realm world and provides the Realm
> +	  Management Interface (RMI) used by a Normal World host to create,
> +	  manage and run protected virtual machines called Realms. The RMM can
> +	  also act as a TSM, as defined by the PCIe TDISP and can manage the
> +	  PCI IDE setup for securing the PCIe links.
> +
> +	  This option builds the host-side RMI support used by KVM to detect a
> +	  compatible RMM, configure it, manage delegated memory and enable
> +	  Realm guests.
> +
> +	  Selecting this option does not by itself make Realm guests available:
> +	  the system must also provide RME-capable hardware and firmware with a
> +	  compatible RMM implementation.
> diff --git a/drivers/firmware/arm_rmm/Makefile b/drivers/firmware/arm_rmm/Makefile
> new file mode 100644
> index 0000000000000..65171988fdcae
> --- /dev/null
> +++ b/drivers/firmware/arm_rmm/Makefile
> @@ -0,0 +1,2 @@
> +
> +obj-$(CONFIG_ARM_RMM_RMI)	= rmi.o
> diff --git a/drivers/firmware/arm_rmm/rmi.c b/drivers/firmware/arm_rmm/rmi.c
> new file mode 100644
> index 0000000000000..3baba931f92e4
> --- /dev/null
> +++ b/drivers/firmware/arm_rmm/rmi.c
> @@ -0,0 +1,109 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2023-2026 ARM Ltd.
> + */
> +
> +#include <linux/cpufeature.h>
> +#include <linux/memblock.h>
> +#include <linux/arm-rmi-cmds.h>
> +#include <linux/slab.h>
> +
> +#include <asm/memory.h>
> +#include <asm/pgtable-hwdef.h>
> +
> +/* RMM defines RmiFeatureRegister0 to RmiFeatureRegister5. */
> +static unsigned long rmi_feat_reg_cache[5] __ro_after_init;
> +

Based on the latest RMM implementation, the highest feature register
index is RMI_FEATURE_REGISTER_4_INDEX. So RmiFeatureRegister5 is out of
the range and the correct comments would be: 'RMM defines RmiFeatureRegister0
to RmiFeatureRegister4', or we could simplify this to:

/* RMM defines 5 feature registers */

> +static int rmi_check_version(void)
> +{
> +	unsigned short version_major, version_minor;
> +	unsigned long host_version = RMI_ABI_VERSION(RMI_ABI_MAJOR_VERSION,
> +						     RMI_ABI_MINOR_VERSION);
> +	unsigned long aa64pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> +	struct arm_smccc_1_2_regs res = {
> +		SMC_RMI_VERSION, host_version,
> +	};
> +
> +	/* If RME isn't supported, then RMI can't be */
> +	if (cpuid_feature_extract_unsigned_field(aa64pfr0, ID_AA64PFR0_EL1_RME_SHIFT) == 0)
> +		return -ENXIO;
> +
> +	rmi_smccc_invoke(&res);
> +	if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
> +		return -ENXIO;
> +
> +	version_major = RMI_ABI_VERSION_GET_MAJOR(res.a1);
> +	version_minor = RMI_ABI_VERSION_GET_MINOR(res.a1);
> +
> +	if (res.a0 != RMI_SUCCESS) {
> +		unsigned short high_version_major, high_version_minor;
> +
> +		high_version_major = RMI_ABI_VERSION_GET_MAJOR(res.a2);
> +		high_version_minor = RMI_ABI_VERSION_GET_MINOR(res.a2);
> +
> +		pr_err("Unsupported RMI ABI (v%d.%d - v%d.%d) we want v%d.%d\n",
> +		       version_major, version_minor,
> +		       high_version_major, high_version_minor,
> +		       RMI_ABI_MAJOR_VERSION,
> +		       RMI_ABI_MINOR_VERSION);
> +		return -ENXIO;
> +	}
> +
> +	pr_info("RMI ABI version %d.%d\n", version_major, version_minor);
> +
> +	return 0;
> +}
> +
> +static int rmi_read_features(void)
> +{
> +	/*
> +	 * Since we've negotiated a compatible version these feature registers
> +	 * should always be available
> +	 */
> +	for (int i = 0; i < ARRAY_SIZE(rmi_feat_reg_cache); i++) {
> +		struct arm_smccc_1_2_regs args = {
> +			SMC_RMI_FEATURES, i,
> +		};
> +
> +		rmi_smccc_invoke(&args);
> +		if (WARN_ON(args.a0 != RMI_SUCCESS))
> +			return -EINVAL;
> +
> +		rmi_feat_reg_cache[i] = args.a1;
> +	}
> +
> +	return 0;
> +}
> +
> +unsigned long rmi_feat_reg(unsigned long index)
> +{
> +	if (WARN_ON(index >= ARRAY_SIZE(rmi_feat_reg_cache)))
> +		return 0;
> +
> +	return rmi_feat_reg_cache[index];
> +}
> +EXPORT_SYMBOL_GPL(rmi_feat_reg);
> +

I guess "unsigned int index" is sufficient for the space as we shouldn't
have that much feature registers :)

> +
> +static int __init arm64_init_rmi(void)
> +{
> +	int ret;
> +
> +	/* If we can't agree on the RMI ABI version, don't proceed further */
> +	ret = rmi_check_version();
> +	if (ret)
> +		return ret;
> +
> +	ret = rmi_read_features();
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> +
> +/*
> + * Note arm64_init_rmi() must be called before kvm_init_rmi() otherwise KVM
> + * will not support realm guests. subsys_initcall() is called before
> + * module_init() (used for KVM) so this is OK.
> + */
> +subsys_initcall(arm64_init_rmi);
> diff --git a/include/linux/arm-rmi-cmds.h b/include/linux/arm-rmi-cmds.h
> new file mode 100644
> index 0000000000000..5c6c563c81555
> --- /dev/null
> +++ b/include/linux/arm-rmi-cmds.h
> @@ -0,0 +1,48 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2026 ARM Ltd.
> + */
> +
> +#ifndef __LINUX_ARM_RMI_CMDS_H_
> +#define __LINUX_ARM_RMI_CMDS_H_
> +
> +#include <linux/arm-smccc-rmi.h>
> +#include <linux/bug.h>
> +#include <linux/processor.h>
> +#include <linux/types.h>
> +
> +#define RMM_BLOCKED_RETRY_COUNT		2
> +/*
> + * rmi_smccc_invoke: Invoke the RMI call and return the results, retrying the
> + * command when status is RMI_BUSY. If we encounter RMI_BLOCKED, we retry
> + * it one more time before we give up. The caller is supposed to handle the
> + * result and reissue if required.
> + *
> + * We don't expect to see RMI_BLOCKED on a practical system, except when
> + * there are parallel requests that results in long standing operation,
> + * with one blocking the other.
> + *
> + * @regs: Input parameters filled in. Updated with the ouptput results
> + * after the call.
> + */
> +static inline void rmi_smccc_invoke(struct arm_smccc_1_2_regs *regs)
> +{
> +	struct arm_smccc_1_2_regs args = *regs;
> +	long status;
> +	int i = 0;
> +
> +	while (i < RMM_BLOCKED_RETRY_COUNT) {
> +		arm_smccc_1_2_invoke(&args, regs);
> +
> +		status = RMI_RESULT_STATUS(regs->a0);
> +		if (status != RMI_BUSY && status != RMI_BLOCKED)
> +			break;
> +		if (status == RMI_BLOCKED)
> +			i++;
> +		cpu_relax();
> +	}
> +}
> +
> +unsigned long rmi_feat_reg(unsigned long index);
> +
> +#endif

As spotted in the previous comments:

#endif /* __LINUX_ARM_RMI_CMDS_H_ */

Thanks,
Gavin