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Subject: Re: [PATCH] KVM: SVM: Move GHCB unmapping to fix RCU warning
From:   Tom Lendacky <thomas.lendacky@amd.com>
To:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Brijesh Singh <brijesh.singh@amd.com>
References: <b2f9b79d15166f2c3e4375c0d9bc3268b7696455.1620332081.git.thomas.lendacky@amd.com>
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Date:   Thu, 6 May 2021 15:31:41 -0500
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On 5/6/21 3:14 PM, Tom Lendacky wrote:
> When an SEV-ES guest is running, the GHCB is unmapped as part of the
> vCPU run support. However, kvm_vcpu_unmap() triggers an RCU dereference
> warning with CONFIG_PROVE_LOCKING=y because the SRCU lock is released
> before invoking the vCPU run support.
> 
> Move the GHCB unmapping into the prepare_guest_switch callback, which is
> invoked while still holding the SRCU lock, eliminating the RCU dereference
> warning.
> 
> Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")

I added the Fixes: tag in case this is to be sent back to stable. But,
5.11 SVM support doesn't have the prepare_guest_switch callback, it was
added in 5.12. This will apply to 5.12 with some fuzz.

Thanks,
Tom

> Reported-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kvm/svm/sev.c | 5 +----
>  arch/x86/kvm/svm/svm.c | 3 +++
>  arch/x86/kvm/svm/svm.h | 1 +
>  3 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index a9d8d6aafdb8..5f70be4e36aa 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2198,7 +2198,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
>  	return -EINVAL;
>  }
>  
> -static void pre_sev_es_run(struct vcpu_svm *svm)
> +void sev_es_unmap_ghcb(struct vcpu_svm *svm)
>  {
>  	if (!svm->ghcb)
>  		return;
> @@ -2234,9 +2234,6 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
>  	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
>  	int asid = sev_get_asid(svm->vcpu.kvm);
>  
> -	/* Perform any SEV-ES pre-run actions */
> -	pre_sev_es_run(svm);
> -
>  	/* Assign the asid allocated with this SEV guest */
>  	svm->asid = asid;
>  
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a7271f31df47..e9f9aacc8f51 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1424,6 +1424,9 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
>  
> +	if (sev_es_guest(vcpu->kvm))
> +		sev_es_unmap_ghcb(svm);
> +
>  	if (svm->guest_state_loaded)
>  		return;
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 84b3133c2251..e44567ceb865 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -581,6 +581,7 @@ void sev_es_init_vmcb(struct vcpu_svm *svm);
>  void sev_es_create_vcpu(struct vcpu_svm *svm);
>  void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
>  void sev_es_prepare_guest_switch(struct vcpu_svm *svm, unsigned int cpu);
> +void sev_es_unmap_ghcb(struct vcpu_svm *svm);
>  
>  /* vmenter.S */
>  
>