Re: [PATCH v14 22/44] arm64: RMI: Handle realm enter/exit

[Steven Price <steven.price@arm.com>]

On 28/05/2026 05:38, Gavin Shan wrote:
> Hi Steve,
> 
> On 5/13/26 11:17 PM, 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>
>> ---
>> Chanegs 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             |  26 ++++-
>>   arch/arm64/kvm/rmi-exit.c        | 186 +++++++++++++++++++++++++++++++
>>   arch/arm64/kvm/rmi.c             |  42 +++++++
>>   5 files changed, 254 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 d99bf4fc3c39..feb534a6678e 100644
>> --- a/arch/arm64/include/asm/kvm_rmi.h
>> +++ b/arch/arm64/include/asm/kvm_rmi.h
>> @@ -84,6 +84,10 @@ 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 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 21d9dfdb1ea0..ed88a203b892 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -1331,6 +1331,9 @@ 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
>> @@ -1378,7 +1381,10 @@ 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++;
>> @@ -1424,7 +1430,9 @@ 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();
> 
> The condition could be posssibly imprecise because ARM_EXCEPTION_CODE(ret)
> can be ARM_EXCEPTION_IRQ even for a REC. So the precise condition would be:
> 
>         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)) {

Good point - I guess this wouldn't have shown up in testing because
there's no harm (other than performance) in the ISB.

>> @@ -1436,8 +1444,13 @@ 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);
>>   @@ -1462,7 +1475,10 @@ 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..e7c51b6cf6ce
>> --- /dev/null
>> +++ b/arch/arm64/kvm/rmi-exit.c
>> @@ -0,0 +1,186 @@
>> +// 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;
>> +}
>> +
> 
> s/rec->run->exit.esr/kvm_vcpu_get_esr(vcpu), rec->run->exit.esr has been
> copied to the storage space pointed by kvm_vcpu_get_esr() in its caller.

Ack

>> +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;
>> +}
>> +
> 
> s/rec->run->exit.esr/kvm_vcpu_get_esr(vcpu)

Ack

>> +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
>> +};
>> +
>> +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;
>> +    }
> 
> I doubt if the flag (REC_ENTER_FLAG_RIPAS_RESPONSE) will be handed over
> to RMM
> since the negative return value forces we're exiting to VMM like QEMU where
> how this problematic case can be handled is TBD.

It's perhaps a bit non-obvious but enter.flags is cleared on the exit.
So even if we return to the VMM the flags will be kept for the next entry.

I agree it is somewhat TBD exactly how this case should be handled -
there's a bunch of "VM did something stupid" cases like this that are a
bit problematic.

Thanks,
Steve

>> +
>> +    /* 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) {
>> +        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;
>> +}
>> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
>> index 353a5ca45e78..d8a5fb12db2d 100644
>> --- a/arch/arm64/kvm/rmi.c
>> +++ b/arch/arm64/kvm/rmi.c
>> @@ -173,6 +173,48 @@ 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:
>> +        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));
>> +    guest_state_exit_irqoff();
>> +
>> +    return ret;
>> +}
>> +
>>   static int kvm_create_rec(struct kvm_vcpu *vcpu)
>>   {
>>       struct user_pt_regs *vcpu_regs = vcpu_gp_regs(vcpu);
> 
> Thanks,
> Gavin
>