Re: [RFC PATCH 13/24] KVM: nSVM: Parameterize svm_flush_tlb_asid() by is_guest_mode

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> svm_flush_tlb_asid() currently operates on the current VMCB. In
> preparation for properly tracking TLB flushes for L1 and L2 ASIDs,
> refactor it to take is_guest_mode and find the proper VMCB. All existing
> callers pass is_guest_mode(vcpu) to maintain existing behavior for now.
> 
> Move the comment about only flushing the current ASID to
> svm_flush_tlb_all(), where it probably should have been anyway, because
> svm_flush_tlb_asid() now flushes a given ASID, not the current ASID.
> 
> Create a svm_flush_tlb_guest() wrapper to use as the flush_tlb_guest()
> callback.
> 
> No functional change intended.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
>  arch/x86/kvm/svm/svm.c | 39 +++++++++++++++++++++++++--------------
>  1 file changed, 25 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 865c5ce4fa473..fb6b9f88a1504 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4016,25 +4016,24 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
>  	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
>  }
>  
> -static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
> +static struct vmcb *svm_get_vmcb(struct vcpu_svm *svm, bool is_guest_mode)
> +{
> +	return is_guest_mode ? svm->nested.vmcb02.ptr : svm->vmcb01.ptr;
> +}

Not sure 100% about this helper, it name might be a bit confusing because
we already have a current vmcb. Maybe add a comment above stating this
this is to get vmcb which might not be currently active?

> +
> +static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu, bool is_guest_mode)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct vmcb *vmcb = svm_get_vmcb(svm, is_guest_mode);
>  
>  	/*
>  	 * Unlike VMX, SVM doesn't provide a way to flush only NPT TLB entries.
>  	 * A TLB flush for the current ASID flushes both "host" and "guest" TLB
>  	 * entries, and thus is a superset of Hyper-V's fine grained flushing.
>  	 */
> -	kvm_hv_vcpu_purge_flush_tlb(vcpu, is_guest_mode(vcpu));
> -
> -	/*
> -	 * Flush only the current ASID even if the TLB flush was invoked via
> -	 * kvm_flush_remote_tlbs().  Although flushing remote TLBs requires all
> -	 * ASIDs to be flushed, KVM uses a single ASID for L1 and L2, and
> -	 * unconditionally does a TLB flush on both nested VM-Enter and nested
> -	 * VM-Exit (via kvm_mmu_reset_context()).
> -	 */
> -	vmcb_set_flush_asid(svm->vmcb);
> +	kvm_hv_vcpu_purge_flush_tlb(vcpu, is_guest_mode);
> +	if (vmcb)
> +		vmcb_set_flush_asid(vmcb);
>  }
>  
>  static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> @@ -4050,7 +4049,7 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
>  	if (svm_hv_is_enlightened_tlb_enabled(vcpu) && VALID_PAGE(root_tdp))
>  		hyperv_flush_guest_mapping(root_tdp);
>  
> -	svm_flush_tlb_asid(vcpu);
> +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
>  }
>  
>  static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
> @@ -4065,7 +4064,14 @@ static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
>  	if (WARN_ON_ONCE(svm_hv_is_enlightened_tlb_enabled(vcpu)))
>  		hv_flush_remote_tlbs(vcpu->kvm);
>  
> -	svm_flush_tlb_asid(vcpu);
> +	/*
> +	 * Flush only the current ASID even if the TLB flush was invoked via
> +	 * kvm_flush_remote_tlbs().  Although flushing remote TLBs requires all
> +	 * ASIDs to be flushed, KVM uses a single ASID for L1 and L2, and
> +	 * unconditionally does a TLB flush on both nested VM-Enter and nested
> +	 * VM-Exit (via kvm_mmu_reset_context()).
> +	 */
> +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
>  }
>  
>  static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> @@ -4075,6 +4081,11 @@ static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
>  	invlpga(gva, svm_get_current_asid(svm));
>  }
>  
> +static void svm_flush_tlb_guest(struct kvm_vcpu *vcpu)
> +{
> +	svm_flush_tlb_asid(vcpu, is_guest_mode(vcpu));
> +}
> +
>  static inline void sync_cr8_to_lapic(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> @@ -5187,7 +5198,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.flush_tlb_all = svm_flush_tlb_all,
>  	.flush_tlb_current = svm_flush_tlb_current,
>  	.flush_tlb_gva = svm_flush_tlb_gva,
> -	.flush_tlb_guest = svm_flush_tlb_asid,
> +	.flush_tlb_guest = svm_flush_tlb_guest,
>  
>  	.vcpu_pre_run = svm_vcpu_pre_run,
>  	.vcpu_run = svm_vcpu_run,


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


Best regards,
	Maxim Levitsky