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Subject: Re: [PATCH v2] KVM: nVMX: Skip IBPB when switching between vmcs01 and
 vmcs02
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
CC:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
        KarimAllah Raslan <karahmed@amazon.de>
References: <20200501163117.4655-1-sean.j.christopherson@intel.com>
From:   Alexander Graf <graf@amazon.com>
Message-ID: <1de7b016-8bc9-23d4-7f8b-145c30d7e58a@amazon.com>
Date:   Mon, 4 May 2020 14:01:46 +0200
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On 01.05.20 18:31, Sean Christopherson wrote:
> =

> Skip the Indirect Branch Prediction Barrier that is triggered on a VMCS
> switch when running with spectre_v2_user=3Don/auto if the switch is
> between two VMCSes in the same guest, i.e. between vmcs01 and vmcs02.
> The IBPB is intended to prevent one guest from attacking another, which
> is unnecessary in the nested case as it's the same guest from KVM's
> perspective.
> =

> This all but eliminates the overhead observed for nested VMX transitions
> when running with CONFIG_RETPOLINE=3Dy and spectre_v2_user=3Don/auto, whi=
ch
> can be significant, e.g. roughly 3x on current systems.
> =

> Reported-by: Alexander Graf <graf@amazon.com>
> Cc: KarimAllah Raslan <karahmed@amazon.de>
> Cc: stable@vger.kernel.org
> Fixes: 15d45071523d ("KVM/x86: Add IBPB support")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> =

> v2: Pass a boolean to indicate a nested VMCS switch and instead WARN if
>      the buddy VMCS is not already loaded.  [Alex]
> =

> Paolo, feel free to drop the WARN_ON_ONCE() if you think it's overkill.
> I'm 50/50 as to whether it's useful or just a waste of cycles.  Figured
> it'd be easier for you to delete a line of code while applying than to add
> and test a new WARN.

I like the WARN_ON :). It should be almost free during execution, but =

helps us catch problems early.

> =

>   arch/x86/kvm/vmx/nested.c | 3 ++-
>   arch/x86/kvm/vmx/vmx.c    | 7 ++++---
>   arch/x86/kvm/vmx/vmx.h    | 2 +-
>   3 files changed, 7 insertions(+), 5 deletions(-)
> =

> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 2c36f3f53108..b57420f3dd8f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -302,8 +302,9 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, st=
ruct loaded_vmcs *vmcs)
> =

>          cpu =3D get_cpu();
>          prev =3D vmx->loaded_vmcs;
> +       WARN_ON_ONCE(prev->cpu !=3D cpu || prev->vmcs !=3D per_cpu(curren=
t_vmcs, cpu));
>          vmx->loaded_vmcs =3D vmcs;
> -       vmx_vcpu_load_vmcs(vcpu, cpu);
> +       vmx_vcpu_load_vmcs(vcpu, cpu, true);
>          vmx_sync_vmcs_host_state(vmx, prev);
>          put_cpu();
> =

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3ab6ca6062ce..d3d57b7a67bd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1311,7 +1311,7 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu,=
 int cpu)
>                  pi_set_on(pi_desc);
>   }
> =

> -void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu)
> +void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_swit=
ch)
>   {
>          struct vcpu_vmx *vmx =3D to_vmx(vcpu);
>          bool already_loaded =3D vmx->loaded_vmcs->cpu =3D=3D cpu;
> @@ -1336,7 +1336,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int =
cpu)
>          if (per_cpu(current_vmcs, cpu) !=3D vmx->loaded_vmcs->vmcs) {
>                  per_cpu(current_vmcs, cpu) =3D vmx->loaded_vmcs->vmcs;
>                  vmcs_load(vmx->loaded_vmcs->vmcs);
> -               indirect_branch_prediction_barrier();

... however, this really needs an in-code comment to explain why it's =

safe not to flush the branch predictor cache here.


Alex

> +               if (!nested_switch)
> +                       indirect_branch_prediction_barrier();
>          }
> =

>          if (!already_loaded) {
> @@ -1377,7 +1378,7 @@ void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   {
>          struct vcpu_vmx *vmx =3D to_vmx(vcpu);
> =

> -       vmx_vcpu_load_vmcs(vcpu, cpu);
> +       vmx_vcpu_load_vmcs(vcpu, cpu, false);
> =

>          vmx_vcpu_pi_load(vcpu, cpu);
> =

> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index b5e773267abe..fa61dc802183 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -320,7 +320,7 @@ struct kvm_vmx {
>   };
> =

>   bool nested_vmx_allowed(struct kvm_vcpu *vcpu);
> -void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu);
> +void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_swit=
ch);
>   void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
>   int allocate_vpid(void);
>   void free_vpid(int vpid);
> --
> 2.26.0
> =


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