linux-kernel - Re: [PATCH v5 19/43] KVM: arm64: Handle realm MMIO emulation

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Message-ID: <3dd5059b-a42d-496b-8de3-e16012d6dafd@arm.com>
Date: Mon, 7 Oct 2024 11:22:13 +0100
From: Steven Price <steven.price@....com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, kvm@...r.kernel.org,
 kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
 Alexandru Elisei <alexandru.elisei@....com>,
 Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
 linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>
Subject: Re: [PATCH v5 19/43] KVM: arm64: Handle realm MMIO emulation

On 07/10/2024 05:31, Aneesh Kumar K.V wrote:
> Steven Price <steven.price@....com> writes:
> 
>> 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@....com>
>> ---
>> v3: Adapt to previous patch changes
>> ---
>>  arch/arm64/kvm/mmio.c     | 10 +++++++++-
>>  arch/arm64/kvm/rme-exit.c |  6 ++++++
>>  2 files changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
>> index cd6b7b83e2c3..66a838b3776a 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"
>> @@ -90,6 +91,9 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>>  
>>  	vcpu->mmio_needed = 0;
>>  
>> +	if (vcpu_is_rec(vcpu))
>> +		vcpu->arch.rec.run->enter.flags |= REC_ENTER_EMULATED_MMIO;
>> +
>>  	if (!kvm_vcpu_dabt_iswrite(vcpu)) {
>>  		struct kvm_run *run = vcpu->run;
>>  
>> @@ -108,7 +112,11 @@ 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);
>>  	}
>>  
>>  	/*
>>
> 
> Does a kvm_incr_pc(vcpu); make sense for realm guest? Should we do

The PC is ignored when restarting realm guest, so kvm_incr_pr() is
effectively a no-op. But I guess REC_ENTER_EMULATED_MMIO is our way of
signalling to the RMM that it should skip the instruction, so your
proposed patch below makes the code slightly clearer.

Thanks,
Steve

> modified   arch/arm64/kvm/mmio.c
> @@ -91,9 +91,6 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>  
>  	vcpu->mmio_needed = 0;
>  
> -	if (vcpu_is_rec(vcpu))
> -		vcpu->arch.rec.run->enter.flags |= RMI_EMULATED_MMIO;
> -
>  	if (!kvm_vcpu_dabt_iswrite(vcpu)) {
>  		struct kvm_run *run = vcpu->run;
>  
> @@ -123,7 +120,10 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>  	 * 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 |= RMI_EMULATED_MMIO;
> +	else
> +		kvm_incr_pc(vcpu);
>  
>  	return 1;
>  }
> 
> 
> 
>> diff --git a/arch/arm64/kvm/rme-exit.c b/arch/arm64/kvm/rme-exit.c
>> index e96ea308212c..1ddbff123149 100644
>> --- a/arch/arm64/kvm/rme-exit.c
>> +++ b/arch/arm64/kvm/rme-exit.c
>> @@ -25,6 +25,12 @@ 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;
>> +
>> +	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]);
>> +
>>  	return kvm_handle_guest_abort(vcpu);
>>  }
>>  
>> -- 
>> 2.34.1