Messages in this thread | | | Subject | Re: [PATCH v2] KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02 | From | Alexander Graf <> | 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=on/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=y and spectre_v2_user=on/auto, which > 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, struct loaded_vmcs *vmcs) > > cpu = get_cpu(); > prev = vmx->loaded_vmcs; > + WARN_ON_ONCE(prev->cpu != cpu || prev->vmcs != per_cpu(current_vmcs, cpu)); > vmx->loaded_vmcs = 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_switch) > { > struct vcpu_vmx *vmx = to_vmx(vcpu); > bool already_loaded = vmx->loaded_vmcs->cpu == cpu; > @@ -1336,7 +1336,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu) > if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) { > per_cpu(current_vmcs, cpu) = 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 = 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_switch); > 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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