Re: [PATCH v13 20/48] arm64: RMI: Handle realm enter/exit

Suzuki K Poulose
2026-03-20T14:09:55Z

On 18/03/2026 15:53, Steven Price 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.
>
> Signed-off-by: Steven Price <steven.price@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> ---
> 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 | 26 ++++-
> arch/arm64/kvm/rmi-exit.c | 178 +++++++++++++++++++++++++++++++
> arch/arm64/kvm/rmi.c | 43 ++++++++
> 5 files changed, 247 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 4e2c61e71a38..7bec3a3976e7 100644
> --- a/arch/arm64/include/asm/kvm_rmi.h
> +++ b/arch/arm64/include/asm/kvm_rmi.h
> @@ -92,6 +92,10 @@ void kvm_destroy_realm(struct kvm *kvm);
> void kvm_realm_destroy_rtts(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 e17c4077d8e7..4b103bcbe760 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 45eff4c41cde..badb94b398bc 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1311,6 +1311,9 @@ 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);
> +
> /*
> * Preparing the interrupts to be injected also
> * involves poking the GIC, which must be done in a
> @@ -1358,7 +1361,10 @@ 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);
>
> vcpu->mode = OUTSIDE_GUEST_MODE;
> vcpu->stat.exits++;
> @@ -1404,7 +1410,9 @@ 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 (ARM_EXCEPTION_CODE(ret) == ARM_EXCEPTION_IRQ ||
> + (vcpu_is_rec(vcpu) &&
> + vcpu->arch.rec.run->exit.exit_reason == RMI_EXIT_IRQ)) {
> local_irq_enable();
> isb();
> local_irq_disable();
> @@ -1416,8 +1424,13 @@ 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);
> + }
>
> kvm_nested_sync_hwstate(vcpu);
>
> @@ -1442,7 +1455,10 @@ 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);
> }
>
> /* Tell userspace about in-kernel device output levels */
> diff --git a/arch/arm64/kvm/rmi-exit.c b/arch/arm64/kvm/rmi-exit.c
> new file mode 100644
> index 000000000000..f5701153dec0
> --- /dev/null
> +++ b/arch/arm64/kvm/rmi-exit.c
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023 ARM Ltd.
> + */
> +
> +#include <linux/kvm_host.h>
> +#include <kvm/arm_hypercalls.h>
> +#include <kvm/arm_psci.h>
> +
> +#include <asm/rmi_smc.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)
> +{
> + struct realm_rec *rec = &vcpu->arch.rec;
> +
> + vcpu_err(vcpu, "Unhandled exit reason from realm (ESR: %#llx)\n",
> + rec->run->exit.esr);
> + return -ENXIO;
> +}
> +
> +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)
> +{
> + struct realm_rec *rec = &vcpu->arch.rec;
> +
> + vcpu_err(vcpu, "Unhandled instruction abort (ESR: %#llx).\n",
> + rec->run->exit.esr);
> + return -ENXIO;
> +}
> +
> +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 & 1);
> + int ret;
> +
> + if (is_write)
> + vcpu_set_reg(vcpu, rt, rec->run->exit.gprs[0]);

The RMM has been fixed to indicate the correct value in ESR_ELx_SRT. So
this could be :
vcpu_set_reg(vcpu, rt, rec->run->ext.gprs[rt]); ?

> +
> + ret = kvm_handle_sys_reg(vcpu);
> + if (!is_write)
> + rec->run->enter.gprs[0] = vcpu_get_reg(vcpu, rt);

Same here ^

> +
> + 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
> +};
> +
> +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);
> +}
> +
> +/*
> + * 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 && index == 1) {
> + vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
> + return 0;
> + }
> +
> + if (rec_run_ret)
> + return -ENXIO;
> +
> + vcpu->arch.fault.esr_el2 = rec->run->exit.esr;

Even ESR_EL2 is only valid when the exit reason is RMI_EXIT_SYNC or
RMI_EXIT_SERROR.
Doing this unconditional copying is fine, as long as we don't consume
the esr_el2 in exit handling without consulting the exit reason, which
may not be available to the rest of the KVM. It may be safer to set it
to 0 ?

> + 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:
> + return 1;
> + case RMI_EXIT_PSCI:
> + return rec_exit_psci(vcpu);
> + case RMI_EXIT_RIPAS_CHANGE:
> + return rec_exit_ripas_change(vcpu);

RMI_EXIT_SERROR is missing in the list above.

> + }
> +
> + kvm_pr_unimpl("Unsupported exit reason: %u\n",
> + rec->run->exit.exit_reason);

> + vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> + return 0;
> +}
> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
> index 6daf14c4b413..ee8aab098117 100644
> --- a/arch/arm64/kvm/rmi.c
> +++ b/arch/arm64/kvm/rmi.c
> @@ -394,6 +394,49 @@ static int realm_ensure_created(struct kvm *kvm)
> return -ENXIO;
> }
>
> +/*
> + * 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;
> +
> + switch (rec->run->exit.exit_reason) {
> + case RMI_EXIT_HOST_CALL:
> + case RMI_EXIT_PSCI:
> + for (int i = 0; i < REC_RUN_GPRS; i++)
> + rec->run->enter.gprs[i] = vcpu_get_reg(vcpu, i);
> + break;
> + }
> +
> + 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));

In the normal VM case, we try to fixup some of the exits (e.g., GIC
CPUIF register accesses) which may be applicable to Realms. Do we
need such fixups here ? Given the cost of world switch, it is
debatable whether it matters or not.

Suzuki
> + guest_state_exit_irqoff();
> +
> + return ret;
> +}
> +
> static void free_rec_aux(struct page **aux_pages,
> unsigned int num_aux)
> {