Re: [RFC PATCH 06/24] KVM: SEV: Track ASID->vCPU instead of ASID->VMCB

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> SEV currently tracks the ASID to VMCB mapping for each physical CPU.
> This is required to flush the ASID when a new VMCB using the same ASID
> is run on the same CPU. 


> Practically, there is a single VMCB for each
> vCPU using SEV. 

Can you elaborate on this a bit? AFAIK you can't run nested with SEV,
even plain SEV because guest state is encrypted, so for SEV we have
indeed one VMCB per vCPU.

> Furthermore, TLB flushes on nested transitions between
> VMCB01 and VMCB02 are handled separately (see
> nested_svm_transition_tlb_flush()).

Yes, or we can say that for now both VMCBs share the same ASID,
up until later in this patch series.

> 
> In preparation for generalizing the tracking and making the tracking
> more expensive, start tracking the ASID to vCPU mapping instead. This
> will allow for the tracking to be moved to a cheaper code path when
> vCPUs are switched.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
>  arch/x86/kvm/svm/sev.c | 12 ++++++------
>  arch/x86/kvm/svm/svm.c |  2 +-
>  arch/x86/kvm/svm/svm.h |  4 ++--
>  3 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d613f81addf1c..ddb4d5b211ed7 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -240,7 +240,7 @@ static void sev_asid_free(struct kvm_sev_info *sev)
>  
>  	for_each_possible_cpu(cpu) {
>  		sd = per_cpu_ptr(&svm_data, cpu);
> -		sd->sev_vmcbs[sev->asid] = NULL;
> +		sd->sev_vcpus[sev->asid] = NULL;
>  	}
>  
>  	mutex_unlock(&sev_bitmap_lock);
> @@ -3081,8 +3081,8 @@ int sev_cpu_init(struct svm_cpu_data *sd)
>  	if (!sev_enabled)
>  		return 0;
>  
> -	sd->sev_vmcbs = kcalloc(nr_asids, sizeof(void *), GFP_KERNEL);
> -	if (!sd->sev_vmcbs)
> +	sd->sev_vcpus = kcalloc(nr_asids, sizeof(void *), GFP_KERNEL);
> +	if (!sd->sev_vcpus)
>  		return -ENOMEM;
>  
>  	return 0;
> @@ -3471,14 +3471,14 @@ int pre_sev_run(struct vcpu_svm *svm, int cpu)
>  	/*
>  	 * Flush guest TLB:
>  	 *
> -	 * 1) when different VMCB for the same ASID is to be run on the same host CPU.
> +	 * 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.
>  	 */
> -	if (sd->sev_vmcbs[asid] == svm->vmcb &&
> +	if (sd->sev_vcpus[asid] == &svm->vcpu &&
>  	    svm->vcpu.arch.last_vmentry_cpu == cpu)
>  		return 0;
>  
> -	sd->sev_vmcbs[asid] = svm->vmcb;
> +	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 18bfc3d3f9ba1..1156ca97fd798 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -694,7 +694,7 @@ static void svm_cpu_uninit(int cpu)
>  	if (!sd->save_area)
>  		return;
>  
> -	kfree(sd->sev_vmcbs);
> +	kfree(sd->sev_vcpus);
>  	__free_page(__sme_pa_to_page(sd->save_area_pa));
>  	sd->save_area_pa = 0;
>  	sd->save_area = NULL;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 843a29a6d150e..4ea6c61c3b048 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -340,8 +340,8 @@ struct svm_cpu_data {
>  
>  	struct vmcb *current_vmcb;
>  
> -	/* index = sev_asid, value = vmcb pointer */
> -	struct vmcb **sev_vmcbs;
> +	/* index = sev_asid, value = vcpu pointer */
> +	struct kvm_vcpu **sev_vcpus;
>  };
>  
>  DECLARE_PER_CPU(struct svm_cpu_data, svm_data);


Code itself looks OK, so 

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

Best regards,
	Maxim Levitsky