Re: [PATCH v14 24/44] KVM: arm64: Handle realm MMIO emulation

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

On 28/05/2026 06:03, Gavin Shan wrote:
> Hi Steve,
> 
> On 5/13/26 11:17 PM, Steven Price wrote:
>> MMIO emulation for a realm cannot be done directly with the VM's
>> registers as they are protected from the host. However, for emulatable
>> data aborts, the RMM uses GPRS[0] to provide the read/written value.
>> We can transfer this from/to the equivalent VCPU's register entry and
>> then depend on the generic MMIO handling code in KVM.
>>
>> For a MMIO read, the value is placed in the shared RecExit structure
>> during kvm_handle_mmio_return() rather than in the VCPU's register
>> entry.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>> Changes since v7:
>>   * New comment for rec_exit_sync_dabt() explaining the call to
>>     vcpu_set_reg().
>> Changes since v5:
>>   * Inject SEA to the guest is an emulatable MMIO access triggers a data
>>     abort.
>>   * kvm_handle_mmio_return() - disable kvm_incr_pc() for a REC (as the PC
>>     isn't under the host's control) and move the REC_ENTER_EMULATED_MMIO
>>     flag setting to this location (as that tells the RMM to skip the
>>     instruction).
>> ---
>>   arch/arm64/kvm/inject_fault.c |  4 +++-
>>   arch/arm64/kvm/mmio.c         | 16 ++++++++++++----
>>   arch/arm64/kvm/rmi-exit.c     | 14 ++++++++++++++
>>   3 files changed, 29 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/inject_fault.c b/arch/arm64/kvm/
>> inject_fault.c
>> index 89982bd3345f..6492397b73d7 100644
>> --- a/arch/arm64/kvm/inject_fault.c
>> +++ b/arch/arm64/kvm/inject_fault.c
>> @@ -228,7 +228,9 @@ static void inject_abt32(struct kvm_vcpu *vcpu,
>> bool is_pabt, u32 addr)
>>     static void __kvm_inject_sea(struct kvm_vcpu *vcpu, bool iabt, u64
>> addr)
>>   {
>> -    if (vcpu_el1_is_32bit(vcpu))
>> +    if (unlikely(vcpu_is_rec(vcpu)))
>> +        vcpu->arch.rec.run->enter.flags |= REC_ENTER_FLAG_INJECT_SEA;
>> +    else if (vcpu_el1_is_32bit(vcpu))
>>           inject_abt32(vcpu, iabt, addr);
>>       else
>>           inject_abt64(vcpu, iabt, addr);
>> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
>> index e2285ed8c91d..6a8cb927fcca 100644
>> --- a/arch/arm64/kvm/mmio.c
>> +++ b/arch/arm64/kvm/mmio.c
>> @@ -6,6 +6,7 @@
>>     #include <linux/kvm_host.h>
>>   #include <asm/kvm_emulate.h>
>> +#include <asm/rmi_smc.h>
>>   #include <trace/events/kvm.h>
>>     #include "trace.h"
>> @@ -138,14 +139,21 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>>           trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr,
>>                      &data);
>>           data = vcpu_data_host_to_guest(vcpu, data, len);
>> -        vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data);
>> +
>> +        if (vcpu_is_rec(vcpu))
>> +            vcpu->arch.rec.run->enter.gprs[0] = data;
>> +        else
>> +            vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data);
>>       }
>>         /*
>>        * The MMIO instruction is emulated and should not be re-executed
>>        * in the guest.
>>        */
>> -    kvm_incr_pc(vcpu);
>> +    if (vcpu_is_rec(vcpu))
>> +        vcpu->arch.rec.run->enter.flags |= REC_ENTER_FLAG_EMULATED_MMIO;
>> +    else
>> +        kvm_incr_pc(vcpu);
>>         return 1;
>>   }
>> @@ -167,14 +175,14 @@ int io_mem_abort(struct kvm_vcpu *vcpu,
>> phys_addr_t fault_ipa)
>>        * No valid syndrome? Ask userspace for help if it has
>>        * volunteered to do so, and bail out otherwise.
>>        *
>> -     * In the protected VM case, there isn't much userspace can do
>> +     * In the protected/realm VM case, there isn't much userspace can do
>>        * though, so directly deliver an exception to the guest.
>>        */
>>       if (!kvm_vcpu_dabt_isvalid(vcpu)) {
>>           trace_kvm_mmio_nisv(*vcpu_pc(vcpu), esr,
>>                       kvm_vcpu_get_hfar(vcpu), fault_ipa);
>>   -        if (vcpu_is_protected(vcpu))
>> +        if (vcpu_is_protected(vcpu) || vcpu_is_rec(vcpu))
>>               return kvm_inject_sea_dabt(vcpu, kvm_vcpu_get_hfar(vcpu));
>>             if (test_bit(KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER,
>> diff --git a/arch/arm64/kvm/rmi-exit.c b/arch/arm64/kvm/rmi-exit.c
>> index e7c51b6cf6ce..8ec0d179eba2 100644
>> --- a/arch/arm64/kvm/rmi-exit.c
>> +++ b/arch/arm64/kvm/rmi-exit.c
>> @@ -25,6 +25,20 @@ static int rec_exit_reason_notimpl(struct kvm_vcpu
>> *vcpu)
>>     static int rec_exit_sync_dabt(struct kvm_vcpu *vcpu)
>>   {
>> +    struct realm_rec *rec = &vcpu->arch.rec;
>> +
>> +    /*
>> +     * In the case of a write, copy over gprs[0] to the target GPR,
>> +     * preparing to handle MMIO write fault. The content to be
>> written has
>> +     * been saved to gprs[0] by the RMM (even if another register was
>> used
>> +     * by the guest). In the case of normal memory access this is
>> redundant
>> +     * (the guest will replay the instruction), but the overhead is
>> +     * minimal.
>> +     */
>> +    if (kvm_vcpu_dabt_iswrite(vcpu) && kvm_vcpu_dabt_isvalid(vcpu))
>> +        vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu),
>> +                 rec->run->exit.gprs[0]);
>> +
> 
> { } is needed here.

Indeed - I'm surprised checkpatch didn't manage to flag that. I'll fix.

Thanks,
Steve

>>       return kvm_handle_guest_abort(vcpu);
>>   }
>>   
> 
> Thanks,
> Gavin
>