linux-kernel - Re: [RFC PATCH 04/24] KVM: SVM: Flush everything if FLUSHBYASID is not available

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Message-ID: <9db2d6e6124227fcda51bce5ac3169c3450685fb.camel@redhat.com>
Date: Thu, 03 Apr 2025 16:00:48 -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 04/24] KVM: SVM: Flush everything if FLUSHBYASID is
 not available

On Wed, 2025-03-26 at 19:35 +0000, Yosry Ahmed wrote:
> Currently, if FLUSHBYASID is not available when performing a TLB flush,
> the fallback is decrementing the ASID generation to trigger allocating a
> new ASID. In preparation for using a static ASID per VM, just fallback
> to flushing everything if FLUSHBYASID is not available. This is probably
> worse from a performance perspective, but FLUSHBYASID has been around
> for ~15 years and it's not worth carrying the complexity.
> 
> The fallback logic is moved within vmcb_set_flush_asid(), as more
> callers will be added and will need the fallback as well.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> ---
>  arch/x86/kvm/svm/svm.c | 5 +----
>  arch/x86/kvm/svm/svm.h | 5 ++++-
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 0e302ae9a8435..5f71b125010d9 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4005,10 +4005,7 @@ static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
>  	 * unconditionally does a TLB flush on both nested VM-Enter and nested
>  	 * VM-Exit (via kvm_mmu_reset_context()).
>  	 */
> -	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
> -		vmcb_set_flush_asid(svm->vmcb);
> -	else
> -		svm->current_vmcb->asid_generation--;
> +	vmcb_set_flush_asid(svm->vmcb);
>  }
>  
>  static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index d2c49cbfbf1ca..843a29a6d150e 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -416,7 +416,10 @@ static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
>  
>  static inline void vmcb_set_flush_asid(struct vmcb *vmcb)
>  {
> -	vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> +	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
> +		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> +	else
> +		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;
>  }
>  
>  static inline void vmcb_clr_flush_asid(struct vmcb *vmcb)

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

Best regards,
	Maxim Levitsky