Messages in this thread | | | Subject | Re: [PATCH] KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02 | From | Alexander Graf <> | Date | Thu, 30 Apr 2020 23:22:20 +0200 |
| |
On 30.04.20 22:41, 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>
I can confirm that with kvm-unit-test's vmcall benchmark, the patch does make a big difference:
BEFORE: vmcall 33488 AFTER: vmcall 14898
So we're at least getting a good chunk of performance back :)
> --- > arch/x86/kvm/vmx/nested.c | 2 +- > arch/x86/kvm/vmx/vmx.c | 12 ++++++++---- > arch/x86/kvm/vmx/vmx.h | 3 ++- > 3 files changed, 11 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index 2c36f3f53108..1a02bdfeeb2b 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -303,7 +303,7 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs) > cpu = get_cpu(); > prev = vmx->loaded_vmcs; > vmx->loaded_vmcs = vmcs; > - vmx_vcpu_load_vmcs(vcpu, cpu); > + vmx_vcpu_load_vmcs(vcpu, cpu, prev); > 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..818dd8ba5e9f 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -1311,10 +1311,12 @@ 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, > + struct loaded_vmcs *buddy) > { > struct vcpu_vmx *vmx = to_vmx(vcpu); > bool already_loaded = vmx->loaded_vmcs->cpu == cpu; > + struct vmcs *prev; > > if (!already_loaded) { > loaded_vmcs_clear(vmx->loaded_vmcs); > @@ -1333,10 +1335,12 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu) > local_irq_enable(); > } > > - if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) { > + prev = per_cpu(current_vmcs, cpu); > + if (prev != vmx->loaded_vmcs->vmcs) { > per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs; > vmcs_load(vmx->loaded_vmcs->vmcs); > - indirect_branch_prediction_barrier(); > + if (!buddy || buddy->vmcs != prev) > + indirect_branch_prediction_barrier();
I fail to understand the logic here though. What exactly are you trying to catch? We only do the barrier when the current_vmcs as loaded by vmx_vcpu_load_vmcs is different from the vmcs of the context that was issuing the vmcs load.
Isn't this a really complicated way to say "Don't flush for nested"? Why not just make it explicit and pass in a bool that says "nested = true" from vmx_switch_vmcs()? Is there any case I'm missing where that would be unsafe?
Thanks,
Alex
Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
| |