Re: [PATCH v15 14/37] KVM: arm64: CCA: Handle realm enter/exit

[Marc Zyngier <maz@kernel.org>]

On Wed, 15 Jul 2026 15:28:16 +0100,
Steven Price <steven.price@arm.com> wrote:
> 
> Entering a realm is done using a SMC call to the RMM. On exit the
> exit-codes need to be handled slightly differently to the normal KVM
> path so define our own functions for realm enter/exit and hook them
> in if the guest is a realm guest.

What is so different that you need to reinvent the wheel?

> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> ---
> Changes since v13:
>  * The RMM is now required to provide an ESR value with the correct
>    information to emulate MMIO, so we no longer need to hardcode 0s in
>    rec_exit_sys_reg().
>  * The PSCI changes mean that there is a potential race when turning on
>    a VCPU which can cause a RMI_ERROR_REC return. Exit to user space
>    with -EAGAIN in this case.
> Changes since v12:
>  * Call guest_state_{enter,exit}_irqoff() around rmi_rec_enter().
>  * Add handling of the IRQ exception case where IRQs need to be briefly
>    enabled before exiting guest timing.
> Changes since v8:
>  * Introduce kvm_rec_pre_enter() called before entering an atomic
>    section to handle operations that might require memory allocation
>    (specifically completing a RIPAS change introduced in a later patch).
>  * Updates to align with upstream changes to hpfar_el2 which now (ab)uses
>    HPFAR_EL2_NS as a valid flag.
>  * Fix exit reason when racing with PSCI shutdown to return
>    KVM_EXIT_SHUTDOWN rather than KVM_EXIT_UNKNOWN.
> Changes since v7:
>  * A return of 0 from kvm_handle_sys_reg() doesn't mean the register has
>    been read (although that can never happen in the current code). Tidy
>    up the condition to handle any future refactoring.
> Changes since v6:
>  * Use vcpu_err() rather than pr_err/kvm_err when there is an associated
>    vcpu to the error.
>  * Return -EFAULT for KVM_EXIT_MEMORY_FAULT as per the documentation for
>    this exit type.
>  * Split code handling a RIPAS change triggered by the guest to the
>    following patch.
> Changes since v5:
>  * For a RIPAS_CHANGE request from the guest perform the actual RIPAS
>    change on next entry rather than immediately on the exit. This allows
>    the VMM to 'reject' a RIPAS change by refusing to continue
>    scheduling.
> Changes since v4:
>  * Rename handle_rme_exit() to handle_rec_exit()
>  * Move the loop to copy registers into the REC enter structure from the
>    to rec_exit_handlers callbacks to kvm_rec_enter(). This fixes a bug
>    where the handler exits to user space and user space wants to modify
>    the GPRS.
>  * Some code rearrangement in rec_exit_ripas_change().
> Changes since v2:
>  * realm_set_ipa_state() now provides an output parameter for the
>    top_iap that was changed. Use this to signal the VMM with the correct
>    range that has been transitioned.
>  * Adapt to previous patch changes.
> ---
>  arch/arm64/include/asm/kvm_rmi.h |   4 +
>  arch/arm64/kvm/Makefile          |   2 +-
>  arch/arm64/kvm/arm.c             |  25 ++++-
>  arch/arm64/kvm/rmi-exit.c        | 182 +++++++++++++++++++++++++++++++
>  arch/arm64/kvm/rmi.c             |  42 +++++++
>  5 files changed, 249 insertions(+), 6 deletions(-)
>  create mode 100644 arch/arm64/kvm/rmi-exit.c
> 
> diff --git a/arch/arm64/include/asm/kvm_rmi.h b/arch/arm64/include/asm/kvm_rmi.h
> index ccd09d55c69f..b1e4cf0f6803 100644
> --- a/arch/arm64/include/asm/kvm_rmi.h
> +++ b/arch/arm64/include/asm/kvm_rmi.h
> @@ -100,6 +100,10 @@ void kvm_destroy_realm(struct kvm *kvm);
>  int kvm_realm_teardown_stage2(struct kvm *kvm);
>  void kvm_destroy_rec(struct kvm_vcpu *vcpu);
>  
> +int kvm_rec_enter(struct kvm_vcpu *vcpu);
> +int kvm_rec_pre_enter(struct kvm_vcpu *vcpu);
> +int handle_rec_exit(struct kvm_vcpu *vcpu, int rec_run_status);
> +
>  static inline bool kvm_realm_is_private_address(struct realm *realm,
>  						unsigned long addr)
>  {
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index ed3cf30eb06e..4a2d52fdb6a2 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -16,7 +16,7 @@ CFLAGS_handle_exit.o += -Wno-override-init
>  kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
>  	 inject_fault.o va_layout.o handle_exit.o config.o \
>  	 guest.o debug.o reset.o sys_regs.o stacktrace.o \
> -	 vgic-sys-reg-v3.o fpsimd.o pkvm.o rmi.o \
> +	 vgic-sys-reg-v3.o fpsimd.o pkvm.o rmi.o rmi-exit.o \
>  	 arch_timer.o trng.o vmid.o emulate-nested.o nested.o at.o \
>  	 vgic/vgic.o vgic/vgic-init.o \
>  	 vgic/vgic-irqfd.o vgic/vgic-v2.o \
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 2959a1451232..534d33b7c67a 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1341,6 +1341,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		if (ret > 0)
>  			ret = check_vcpu_requests(vcpu);
>  
> +		if (ret > 0 && vcpu_is_rec(vcpu))
> +			ret = kvm_rec_pre_enter(vcpu);
> +

What needs to be done at this stage? Why isn't this handled as a
request? Or similarly to an MMIO return?

>  		/*
>  		 * Preparing the interrupts to be injected also
>  		 * involves poking the GIC, which must be done in a
> @@ -1388,7 +1391,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		trace_kvm_entry(*vcpu_pc(vcpu));
>  		guest_timing_enter_irqoff();
>  
> -		ret = kvm_arm_vcpu_enter_exit(vcpu);
> +		if (vcpu_is_rec(vcpu))
> +			ret = kvm_rec_enter(vcpu);
> +		else
> +			ret = kvm_arm_vcpu_enter_exit(vcpu);

This either sidesteps all the RCU EQS, context management and co, or
duplicates it. Either way, that's no good.
>  
>  		vcpu->mode = OUTSIDE_GUEST_MODE;
>  		vcpu->stat.exits++;
> @@ -1434,7 +1440,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		 * context synchronization event) is necessary to ensure that
>  		 * pending interrupts are taken.
>  		 */
> -		if (ARM_EXCEPTION_CODE(ret) == ARM_EXCEPTION_IRQ) {
> +		if ((!vcpu_is_rec(vcpu) && ARM_EXCEPTION_CODE(ret) == ARM_EXCEPTION_IRQ) ||
> +		    (vcpu_is_rec(vcpu) && vcpu->arch.rec.run->exit.exit_reason == RMI_EXIT_IRQ)) {

No. Please adapt the CCA code so that what it returns has the exact
same format as the non-CCA code, having the vcpu state placed where it
matters.

>  			local_irq_enable();
>  			isb();
>  			local_irq_disable();
> @@ -1446,8 +1453,13 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  
>  		trace_kvm_exit(ret, kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu));
>  
> -		/* Exit types that need handling before we can be preempted */
> -		handle_exit_early(vcpu, ret);
> +		if (!vcpu_is_rec(vcpu)) {
> +			/*
> +			 * Exit types that need handling before we can be
> +			 * preempted
> +			 */
> +			handle_exit_early(vcpu, ret);
> +		}

Same thing.

>  
>  		kvm_nested_sync_hwstate(vcpu);
>  
> @@ -1472,7 +1484,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  			ret = ARM_EXCEPTION_IL;
>  		}
>  
> -		ret = handle_exit(vcpu, ret);
> +		if (vcpu_is_rec(vcpu))
> +			ret = handle_rec_exit(vcpu, ret);
> +		else
> +			ret = handle_exit(vcpu, ret);

And same thing here as well.

>  	}
>  
>  	if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
> diff --git a/arch/arm64/kvm/rmi-exit.c b/arch/arm64/kvm/rmi-exit.c
> new file mode 100644
> index 000000000000..973250563d7b
> --- /dev/null
> +++ b/arch/arm64/kvm/rmi-exit.c
> @@ -0,0 +1,182 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023-2026 ARM Ltd.
> + */
> +
> +#include <linux/kvm_host.h>
> +#include <kvm/arm_hypercalls.h>
> +#include <kvm/arm_psci.h>
> +
> +#include <linux/arm-smccc-rmi.h>
> +#include <asm/kvm_emulate.h>
> +#include <asm/kvm_rmi.h>
> +#include <asm/kvm_mmu.h>
> +
> +typedef int (*exit_handler_fn)(struct kvm_vcpu *vcpu);
> +
> +static int rec_exit_reason_notimpl(struct kvm_vcpu *vcpu)
> +{
> +	vcpu_err(vcpu, "Unhandled exit reason from realm (ESR: %#llx)\n",
> +		 kvm_vcpu_get_esr(vcpu));
> +	return -ENXIO;
> +}

This should mark the VM dead.

> +
> +static int rec_exit_sync_dabt(struct kvm_vcpu *vcpu)
> +{
> +	return kvm_handle_guest_abort(vcpu);
> +}
> +
> +static int rec_exit_sync_iabt(struct kvm_vcpu *vcpu)
> +{
> +	vcpu_err(vcpu, "Unhandled instruction abort (ESR: %#llx).\n",
> +		 kvm_vcpu_get_esr(vcpu));
> +	return -ENXIO;
> +}

If you can't handle it, why do you specify a handler? isn't the
default handler enough?

> +
> +static int rec_exit_sys_reg(struct kvm_vcpu *vcpu)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +	unsigned long esr = kvm_vcpu_get_esr(vcpu);
> +	int rt = kvm_vcpu_sys_get_rt(vcpu);
> +	bool is_write = (esr & ESR_ELx_SYS64_ISS_DIR_MASK) == ESR_ELx_SYS64_ISS_DIR_WRITE;
> +	int ret;
> +
> +	if (is_write)
> +		vcpu_set_reg(vcpu, rt, rec->run->exit.gprs[rt]);
> +
> +	ret = kvm_handle_sys_reg(vcpu);
> +	if (!is_write)
> +		rec->run->enter.gprs[rt] = vcpu_get_reg(vcpu, rt);
> +
> +	return ret;
> +}
> +
> +static exit_handler_fn rec_exit_handlers[] = {
> +	[0 ... ESR_ELx_EC_MAX]	= rec_exit_reason_notimpl,
> +	[ESR_ELx_EC_SYS64]	= rec_exit_sys_reg,
> +	[ESR_ELx_EC_DABT_LOW]	= rec_exit_sync_dabt,
> +	[ESR_ELx_EC_IABT_LOW]	= rec_exit_sync_iabt
> +};

Frankly, none of that should exist. You should simply populate the
state in the vcpu, and let the normal handlers run.

> +
> +static int rec_exit_psci(struct kvm_vcpu *vcpu)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +	int i;
> +
> +	for (i = 0; i < REC_RUN_GPRS; i++)
> +		vcpu_set_reg(vcpu, i, rec->run->exit.gprs[i]);
> +
> +	return kvm_smccc_call_handler(vcpu);
> +}
> +
> +static int rec_exit_ripas_change(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	struct realm *realm = &kvm->arch.realm;
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +	unsigned long base = rec->run->exit.ripas_base;
> +	unsigned long top = rec->run->exit.ripas_top;
> +	unsigned long ripas = rec->run->exit.ripas_value;
> +
> +	if (!kvm_realm_is_private_address(realm, base) ||
> +	    !kvm_realm_is_private_address(realm, top - 1)) {
> +		vcpu_err(vcpu, "Invalid RIPAS_CHANGE for %#lx - %#lx, ripas: %#lx\n",
> +			 base, top, ripas);
> +		/* Set RMI_REJECT bit */
> +		rec->run->enter.flags = REC_ENTER_FLAG_RIPAS_RESPONSE;
> +		return -EINVAL;
> +	}
> +
> +	/* Exit to VMM, the actual RIPAS change is done on next entry */
> +	kvm_prepare_memory_fault_exit(vcpu, base, top - base, false, false,
> +				      ripas == RMI_RAM);
> +
> +	/*
> +	 * KVM_EXIT_MEMORY_FAULT requires an return code of -EFAULT, see the
> +	 * API documentation
> +	 */
> +	return -EFAULT;
> +}
> +
> +static void update_arch_timer_irq_lines(struct kvm_vcpu *vcpu)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +
> +	__vcpu_assign_sys_reg(vcpu, CNTV_CTL_EL0, rec->run->exit.cntv_ctl);
> +	__vcpu_assign_sys_reg(vcpu, CNTV_CVAL_EL0, rec->run->exit.cntv_cval);
> +	__vcpu_assign_sys_reg(vcpu, CNTP_CTL_EL0, rec->run->exit.cntp_ctl);
> +	__vcpu_assign_sys_reg(vcpu, CNTP_CVAL_EL0, rec->run->exit.cntp_cval);
> +
> +	kvm_realm_timers_update(vcpu);

Why is that saved to memory? The expectations are that the HW timers
are live when the guest state is loaded.

> +}
> +
> +/*
> + * Return > 0 to return to guest, < 0 on error, 0 (and set exit_reason) on
> + * proper exit to userspace.
> + */
> +int handle_rec_exit(struct kvm_vcpu *vcpu, int rec_run_ret)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +	u8 esr_ec = ESR_ELx_EC(rec->run->exit.esr);
> +	unsigned long status, index;
> +
> +	status = RMI_RETURN_STATUS(rec_run_ret);
> +	index = RMI_RETURN_INDEX(rec_run_ret);
> +
> +	/*
> +	 * If a PSCI_SYSTEM_OFF request raced with a vcpu executing, we might
> +	 * see the following status code and index indicating an attempt to run
> +	 * a REC when the RD state is SYSTEM_OFF.  In this case, we just need to
> +	 * return to user space which can deal with the system event or will try
> +	 * to run the KVM VCPU again, at which point we will no longer attempt
> +	 * to enter the Realm because we will have a sleep request pending on
> +	 * the VCPU as a result of KVM's PSCI handling.
> +	 */
> +	if (status == RMI_ERROR_REALM) {
> +		vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
> +		return 0;
> +	}
> +
> +	/*
> +	 * If a VCPU has been turned on, but the REC state hasn't been updated
> +	 * we may experience RMI_ERROR_REC. Exit to the userspace with -EAGAIN
> +	 * for a retry.
> +	 */
> +	if (status == RMI_ERROR_REC)
> +		return -EAGAIN;
> +	if (rec_run_ret)
> +		return -ENXIO;
> +
> +	vcpu->arch.fault.esr_el2 = rec->run->exit.esr;
> +	vcpu->arch.fault.far_el2 = rec->run->exit.far;
> +	/* HPFAR_EL2 is only valid for RMI_EXIT_SYNC */
> +	vcpu->arch.fault.hpfar_el2 = 0;
> +
> +	update_arch_timer_irq_lines(vcpu);
> +
> +	/* Reset the emulation flags for the next run of the REC */
> +	rec->run->enter.flags = 0;
> +
> +	switch (rec->run->exit.exit_reason) {
> +	case RMI_EXIT_SYNC:
> +		/*
> +		 * HPFAR_EL2_NS is hijacked to indicate a valid HPFAR value,
> +		 * see __get_fault_info()
> +		 */
> +		vcpu->arch.fault.hpfar_el2 = rec->run->exit.hpfar | HPFAR_EL2_NS;
> +		return rec_exit_handlers[esr_ec](vcpu);
> +	case RMI_EXIT_IRQ:
> +	case RMI_EXIT_FIQ:
> +	case RMI_EXIT_SERROR:
> +		return 1;
> +	case RMI_EXIT_PSCI:
> +		return rec_exit_psci(vcpu);
> +	case RMI_EXIT_RIPAS_CHANGE:
> +		return rec_exit_ripas_change(vcpu);
> +	}
> +
> +	kvm_pr_unimpl("Unsupported exit reason: %u\n",
> +		      rec->run->exit.exit_reason);
> +	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> +	return 0;
> +}

-ETOOMUCHCODE. Please rework this so that this is modelled after the
 existing KVM code. Extra handling (such as the other RMI_EXIT*
 reasons) can be treated as if they were an HVC call from the guest.

> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
> index aa97ad5fd7e9..430b82c02d15 100644
> --- a/arch/arm64/kvm/rmi.c
> +++ b/arch/arm64/kvm/rmi.c
> @@ -203,6 +203,48 @@ int kvm_realm_teardown_stage2(struct kvm *kvm)
>  	return realm_destroy_rtts(kvm);
>  }
>  
> +/*
> + * kvm_rec_pre_enter - Complete operations before entering a REC
> + *
> + * Some operations require work to be completed before entering a realm. That
> + * work may require memory allocation so cannot be done in the kvm_rec_enter()
> + * call.
> + *
> + * Return: 1 if we should enter the guest
> + *	   0 if we should exit to userspace
> + *	   < 0 if we should exit to userspace, where the return value indicates
> + *	   an error
> + */
> +int kvm_rec_pre_enter(struct kvm_vcpu *vcpu)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +
> +	if (kvm_realm_state(vcpu->kvm) != REALM_STATE_ACTIVE)
> +		return -EINVAL;

Do you really need to check for this in the run loop?

> +
> +	switch (rec->run->exit.exit_reason) {
> +	case RMI_EXIT_HOST_CALL:
> +		for (int i = 0; i < REC_RUN_GPRS; i++)
> +			rec->run->enter.gprs[i] = vcpu_get_reg(vcpu, i);
> +		break;

If this only happens on returning to the guest from a host call, this
definitely can be done as a request.

> +	}
> +
> +	return 1;
> +}
> +
> +int noinstr kvm_rec_enter(struct kvm_vcpu *vcpu)
> +{
> +	struct realm_rec *rec = &vcpu->arch.rec;
> +	int ret;
> +
> +	guest_state_enter_irqoff();
> +	ret = rmi_rec_enter(virt_to_phys(rec->rec_page),
> +			    virt_to_phys(rec->run));

Why don't you keep the PAs around instead? This should be merged into
kvm_arm_vcpu_enter_exit().


> +	guest_state_exit_irqoff();
> +
> +	return ret;
> +}
> +
>  static int __maybe_unused kvm_create_rec(struct kvm_vcpu *vcpu)
>  {
>  	struct user_pt_regs *vcpu_regs = vcpu_gp_regs(vcpu);

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.