Re: [PATCH v15 16/37] KVM: arm64: CCA: Handle realm MMIO emulation

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

On 03/08/2026 12:27, Marc Zyngier wrote:
> On Wed, 15 Jul 2026 15:28:18 +0100,
> Steven Price <steven.price@arm.com> 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     | 15 +++++++++++++++
>>  3 files changed, 30 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);
> 
> Why don't you let inject_dabt64() do its job and reconcile the REC
> stuff at run time?

Ok, one wrinkle is the __kvm_adjust_pc() path. But I can skip that and
leave the PENDING_EXCEPTION flag set on return to user space. That way
the flag is still set for the next entry and we can handle the RMM
specific parts there.

>> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
>> index e2285ed8c91d..a8c125205695 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 <linux/arm-smccc-rmi.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);
> 
> This is yet another example of things I do not want to see. KVM works
> on the GPRs described in the vcpu structure. And that's it.
> 
> So let the ESR information be correct for CCA, the data being written
> back to x0, and once you're ready to enter the guest again, copy
> whatever you need into the CCA data structure.
> 
> But not any of this stuff.

Yes this is a bit silly. A while ago the RMM wasn't populating the ESR
information correctly which is where all these gprs[0] hacks came from.
That's been fixed and it should be possible to do this on entry.

>>  	}
>>  
>>  	/*
>>  	 * 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);
> 
> Same thing. kvm_incr_pc() sets a flag. Use that to reconcile the state
> with CCA.

It's not an ideal flag for this use (it's set in other places). But I
think it should be sufficient along with checking other state to
identify this condition.

Thanks,
Steve