linux-kernel - Re: [PATCH 2/2] KVM:SVM: Mask SEV encryption bit from CR3 reserved bits

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Date:   Thu, 12 Nov 2020 09:32:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Babu Moger <babu.moger@....com>
Cc:     junaids@...gle.com, wanpengli@...cent.com, kvm@...r.kernel.org,
        joro@...tes.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        sean.j.christopherson@...el.com, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, tglx@...utronix.de, vkuznets@...hat.com,
        jmattson@...gle.com
Subject: Re: [PATCH 2/2] KVM:SVM: Mask SEV encryption bit from CR3 reserved
 bits

On 12/11/20 01:28, Babu Moger wrote:
> Add support to the mask_cr3_rsvd_bits() callback to mask the
> encryption bit from the CR3 value when SEV is enabled.
> 
> Additionally, cache the encryption mask for quick access during
> the check.
> 
> Fixes: a780a3ea628268b2 ("KVM: X86: Fix reserved bits check for MOV to CR3")
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
>   arch/x86/kvm/svm/svm.c |   11 ++++++++++-
>   arch/x86/kvm/svm/svm.h |    3 +++
>   2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a491a47d7f5c..c2b1e52810c6 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3741,6 +3741,7 @@ static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
>   static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct kvm_cpuid_entry2 *best;
>   
>   	vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
>   				    boot_cpu_has(X86_FEATURE_XSAVE) &&
> @@ -3771,6 +3772,12 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>   	if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
>   		kvm_request_apicv_update(vcpu->kvm, false,
>   					 APICV_INHIBIT_REASON_NESTED);
> +
> +	best = kvm_find_cpuid_entry(vcpu, 0x8000001F, 0);
> +	if (best)
> +		svm->sev_enc_mask = ~(1UL << (best->ebx & 0x3f));
> +	else
> +		svm->sev_enc_mask = ~0UL;
>   }
>   
>   static bool svm_has_wbinvd_exit(void)
> @@ -4072,7 +4079,9 @@ static void enable_smi_window(struct kvm_vcpu *vcpu)
>   
>   static unsigned long svm_mask_cr3_rsvd_bits(struct kvm_vcpu *vcpu, unsigned long cr3)
>   {
> -	return cr3;
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +
> +	return sev_guest(vcpu->kvm) ? (cr3 & svm->sev_enc_mask) : cr3;
>   }
>   
>   static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int insn_len)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 1d853fe4c778..57a36645a0e4 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -152,6 +152,9 @@ struct vcpu_svm {
>   	u64 *avic_physical_id_cache;
>   	bool avic_is_running;
>   
> +	/* SEV Memory encryption mask */
> +	unsigned long sev_enc_mask;
> +
>   	/*
>   	 * Per-vcpu list of struct amd_svm_iommu_ir:
>   	 * This is used mainly to store interrupt remapping information used
> 

Instead of adding a new callback, you can add a field to struct 
kvm_vcpu_arch:

  	if (is_long_mode(vcpu) &&
-	    (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 63)))
+	    (cr3 & vcpu->arch.cr3_lm_rsvd_bits))

Set it in kvm_vcpu_after_set_cpuid, and clear the memory encryption bit 
in kvm_x86_ops.vcpu_after_set_cpuid.

Paolo