Re: [RFC PATCH 11/24] KVM: nSVM: Use a separate ASID for nested guests

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> The per-VM ASID is currently shared by both L1 and L2 guests. That ASID
> is currently flushed on every transition between L1 and L2.
> 
> Allocate and track a separate ASID per-VM for nested guests. This is in
> preparation for doing fine-grained TLB flushes on nested transitions
> instead of unconditional full flushes.
> 
> Nested ASIDs are still not fully maintained (e.g. a remote flush will
> only flush the current ASID), so keep the TLB flush on every transition
> until this is sorted out in following changes.
> 
> Add a helper to get the ASID associated with a specific VMCB and use it
> instead of directly reading the VM's ASID. This transparently uses L2's
> ASID when an L2 guest is being run.
> 
> L1's ASID is flushed on KVM_REQ_TLB_FLUSH_GUEST if it is the active
> context, so remove the TODO in nested_svm_transition_tlb_flush() about
> it.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
>  arch/x86/kvm/svm/nested.c |  8 ++++++--
>  arch/x86/kvm/svm/svm.c    | 13 +++++++++++--
>  arch/x86/kvm/svm/svm.h    |  3 ++-
>  3 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 81184b2fb27fd..75223869aa8c6 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -495,7 +495,6 @@ static void nested_svm_transition_tlb_flush(struct kvm_vcpu *vcpu)
>  	 *  - Honor L1's request to flush an ASID on nested VMRUN
>  	 *  - Sync nested NPT MMU on VMRUN that flushes L2's ASID[*]
>  	 *  - Don't crush a pending TLB flush in vmcb02 on nested VMRUN
> -	 *  - Flush L1's ASID on KVM_REQ_TLB_FLUSH_GUEST
>  	 *
>  	 * [*] Unlike nested EPT, SVM's ASID management can invalidate nested
>  	 *     NPT guest-physical mappings on VMRUN.
> @@ -677,7 +676,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
>  	vmcb02->control.nested_ctl = vmcb01->control.nested_ctl;
>  	vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
>  	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
> -	vmcb02->control.asid = svm_asid(vcpu->kvm);
> +	vmcb02->control.asid = svm_nested_asid(vcpu->kvm);
>  
>  	/* Also overwritten later if necessary.  */
>  	vmcb_clr_flush_asid(vmcb02);
> @@ -1179,6 +1178,7 @@ static void nested_svm_triple_fault(struct kvm_vcpu *vcpu)
>  
>  int svm_allocate_nested(struct vcpu_svm *svm)
>  {
> +	struct kvm_svm *kvm_svm = to_kvm_svm(svm->vcpu.kvm);
>  	struct page *vmcb02_page;
>  
>  	if (svm->nested.initialized)
> @@ -1196,6 +1196,10 @@ int svm_allocate_nested(struct vcpu_svm *svm)
>  	svm_vcpu_init_msrpm(&svm->vcpu, svm->nested.msrpm);
>  
>  	svm->nested.initialized = true;
> +
> +	if (!kvm_svm->nested_asid)
> +		kvm_svm->nested_asid = kvm_svm->asid;

Nitpick: maybe put nested_asid into .nested struct as well?
I don't have a strong option on this, feel free to leave it where it is now.


> +
>  	return 0;
>  
>  err_free_vmcb02:
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f028d006f69dc..e664d8428c792 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1225,17 +1225,26 @@ static inline void init_vmcb_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> -unsigned int svm_asid(struct kvm *kvm)
> +unsigned int svm_nested_asid(struct kvm *kvm)
> +{
> +	return to_kvm_svm(kvm)->nested_asid;
> +}

It might also make sense to add WARN_ON_ONCE(!svm->nested.initialized) here, just in case.

> +
> +static unsigned int svm_asid(struct kvm *kvm)
>  {
>  	return to_kvm_svm(kvm)->asid;
>  }
>  
>  static unsigned int svm_get_current_asid(struct vcpu_svm *svm)
>  {
> -	struct kvm *kvm = svm->vcpu.kvm;
> +	struct kvm_vcpu *vcpu = &svm->vcpu;
> +	struct kvm *kvm = vcpu->kvm;
>  
>  	if (sev_guest(kvm))
>  		return sev_get_asid(kvm);
> +	if (is_guest_mode(vcpu))
> +		return svm_nested_asid(kvm);
> +	WARN_ON_ONCE(svm->current_vmcb != &svm->vmcb01);
>  	return svm_asid(kvm);
>  }
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 436b7e83141b9..e67e3a64e92f7 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -118,6 +118,7 @@ struct kvm_svm {
>  	struct kvm kvm;
>  
>  	unsigned int asid;
> +	unsigned int nested_asid;
>  
>  	/* Struct members for AVIC */
>  	u32 avic_vm_id;
> @@ -651,7 +652,7 @@ void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode,
>  				     int trig_mode, int vec);
>  bool svm_register_asid(unsigned int asid);
>  void svm_unregister_asid(unsigned int asid);
> -unsigned int svm_asid(struct kvm *kvm);
> +unsigned int svm_nested_asid(struct kvm *kvm);
>  
>  /* nested.c */
>  


Overall looks good,

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

Best regards,
	Maxim Levitsky