Re: [RFC PATCH 07/24] KVM: SEV: Track ASID->vCPU on vCPU load

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> Check for changes in the ASID to vCPU mapping on vCPU load instead of
> doing it on vCPU run. This should be sufficient and more efficient, and
> is needed to allow generalizing the tracking and making it more
> expensive.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
>  arch/x86/kvm/svm/sev.c | 13 ++++---------
>  arch/x86/kvm/svm/svm.c | 13 +++++++++++++
>  arch/x86/kvm/svm/svm.h |  1 +
>  3 files changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index ddb4d5b211ed7..3ef0dfdbb34d2 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -224,7 +224,7 @@ static int sev_asid_new(struct kvm_sev_info *sev)
>  	return ret;
>  }
>  
> -static unsigned int sev_get_asid(struct kvm *kvm)
> +unsigned int sev_get_asid(struct kvm *kvm)
>  {
>  	return to_kvm_sev_info(kvm)->asid;
>  }
> @@ -3453,7 +3453,6 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm)
>  
>  int pre_sev_run(struct vcpu_svm *svm, int cpu)
>  {
> -	struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, cpu);
>  	struct kvm *kvm = svm->vcpu.kvm;
>  	unsigned int asid = sev_get_asid(kvm);
>  
> @@ -3469,16 +3468,12 @@ int pre_sev_run(struct vcpu_svm *svm, int cpu)
>  	svm->asid = asid;
>  
>  	/*
> -	 * Flush guest TLB:
> -	 *
> -	 * 1) when different vCPU for the same ASID is to be run on the same host CPU.
> -	 * 2) or this VMCB was executed on different host CPU in previous VMRUNs.
> +	 * Flush guest TLB if the VMCB was executed on a differet host CPU in
> +	 * previous VMRUNs.
>  	 */
> -	if (sd->sev_vcpus[asid] == &svm->vcpu &&
> -	    svm->vcpu.arch.last_vmentry_cpu == cpu)
> +	if (svm->vcpu.arch.last_vmentry_cpu == cpu)
>  		return 0;
>  
> -	sd->sev_vcpus[asid] = &svm->vcpu;
>  	vmcb_set_flush_asid(svm->vmcb);
>  	vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
>  	return 0;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 1156ca97fd798..e6e380411fbec 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1554,6 +1554,7 @@ static void svm_prepare_host_switch(struct kvm_vcpu *vcpu)
>  
>  static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  {
> +	unsigned int asid;
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, cpu);
>  
> @@ -1568,6 +1569,18 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	}
>  	if (kvm_vcpu_apicv_active(vcpu))
>  		avic_vcpu_load(vcpu, cpu);
> +
> +	if (sev_guest(vcpu->kvm)) {
> +		/*
> +		 * Flush the TLB when a different vCPU using the same ASID is
> +		 * run on the same CPU.
> +		 */
> +		asid = sev_get_asid(vcpu->kvm);
> +		if (sd->sev_vcpus[asid] != vcpu) {
> +			sd->sev_vcpus[asid] = vcpu;
> +			kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> +		}
> +	}
>  }
>  
>  static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 4ea6c61c3b048..ca38a233fa24c 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -768,6 +768,7 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm);
>  void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
>  void sev_es_prepare_switch_to_guest(struct vcpu_svm *svm, struct sev_es_save_area *hostsa);
>  void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> +unsigned int sev_get_asid(struct kvm *kvm);
>  
>  #ifdef CONFIG_KVM_AMD_SEV
>  int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp);

Makes sense, but I might have missed something.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky