linux-kernel - Re: [RFC PATCH 17/24] KVM: nSVM: Flush both L1 and L2 ASIDs on KVM_REQ_TLB

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Message-ID: <a377b99b3bc3f6417d098d50351915ddb410fb55.camel@redhat.com>
Date: Thu, 03 Apr 2025 16:10:32 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>, Sean Christopherson
 <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>, 
 Vitaly Kuznetsov <vkuznets@...hat.com>, Rik van Riel <riel@...riel.com>,
 Tom Lendacky <thomas.lendacky@....com>,  x86@...nel.org,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 17/24] KVM: nSVM: Flush both L1 and L2 ASIDs on
 KVM_REQ_TLB_FLUSH

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> KVM_REQ_TLB_FLUSH is used to flush all TLB entries for all contexts
> (e.g. in kvm_flush_remote_tlbs()). Flush both L1 and L2 ASIDs in
> svm_flush_tlb_all() to handle it appropriately.
> 
> This is currently not required as nested transitions do unconditional
> TLB flushes, but this is a step toward eliminating that.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> ---
>  arch/x86/kvm/svm/nested.c |  1 -
>  arch/x86/kvm/svm/svm.c    | 10 ++--------
>  2 files changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index c336ab63c6da3..56a4ff480bb3d 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -491,7 +491,6 @@ static void nested_svm_entry_tlb_flush(struct kvm_vcpu *vcpu)
>  	 * TODO: optimize unconditional TLB flush/MMU sync.  A partial list of
>  	 * things to fix before this can be conditional:
>  	 *
> -	 *  - Flush TLBs for both L1 and L2 remote TLB flush
>  	 *  - 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
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index fb6b9f88a1504..4cad1085936bb 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4064,14 +4064,8 @@ 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);
>  
> -	/*
> -	 * 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));
> +	svm_flush_tlb_asid(vcpu, false);
> +	svm_flush_tlb_asid(vcpu, true);
>  }
>  
>  static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)

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

Best regards,
	Maxim Levitsky