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

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Message-ID: <710c066e-75fa-4f12-b27a-c8948d02bb4b@arm.com>
Date: Thu, 17 Oct 2024 12:59:06 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Steven Price <steven.price@....com>, 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>, 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>, "Aneesh Kumar K . V"
 <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v5 19/43] KVM: arm64: Handle realm MMIO emulation

On 04/10/2024 16:27, 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@....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;

Should we additionally handle injecting an abort if there was no valid
syndrome information ? Like we do for protected VMs and normal VMs when
userspace doesn't offer to help ?

>   
> @@ -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;

I wonder if we can skip this here and we can sync the "enter.gprs[]"
from vcpu state at rec_enter, similar to what we do for PSCI/HOST call
exits. Also the ESR_ELx_SRT is always x0 for a Realm exit. So, we should
always find the enter.gpr[0] in vcpu.regs[0] at rec_enter.

Suzuki

> +		else
> +			vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data);
>   	}
>   
>   	/*
> 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);
>   }
>