linux-kernel - Re: [RFC PATCH 05/24] KVM: SVM: Flush the ASID when running on a new CPU

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Message-ID: <3ab79e6f97a0a3610c4841e52b6233248d1a76a8.camel@redhat.com>
Date: Thu, 03 Apr 2025 16:00:56 -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 05/24] KVM: SVM: Flush the ASID when running on a
 new CPU

On Wed, 2025-03-26 at 19:36 +0000, Yosry Ahmed wrote:
> Currently, when a vCPU is migrated to a new physical CPU, the ASID
> generation is reset to trigger allocating a new ASID. In preparation for
> using a static ASID per VM, just flush the ASID in this case (falling
> back to flushing everything if FLUSBYASID is not available).
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> ---
>  arch/x86/kvm/svm/svm.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 5f71b125010d9..18bfc3d3f9ba1 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3626,12 +3626,12 @@ static int pre_svm_run(struct kvm_vcpu *vcpu)
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
>  	/*
> -	 * If the previous vmrun of the vmcb occurred on a different physical
> -	 * cpu, then mark the vmcb dirty and assign a new asid.  Hardware's
> -	 * vmcb clean bits are per logical CPU, as are KVM's asid assignments.
> +	 * If the previous VMRUN of the VMCB occurred on a different physical
> +	 * CPU, then mark the VMCB dirty and flush the ASID.  Hardware's
> +	 * VMCB clean bits are per logical CPU, as are KVM's ASID assignments.
>  	 */
>  	if (unlikely(svm->current_vmcb->cpu != vcpu->cpu)) {
> -		svm->current_vmcb->asid_generation = 0;
> +		vmcb_set_flush_asid(svm->vmcb);
>  		vmcb_mark_all_dirty(svm->vmcb);
>  		svm->current_vmcb->cpu = vcpu->cpu;
>          }

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

Best regards,
	Maxim Levitsky