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    SubjectRe: [PATCH 4/7] kvm: vmx: Set IBPB when running a different VCPU
    Shouldn't there be an IBPB on *any* context switch away from a VCPU
    thread, even if it is to a non-VCPU thread?

    On Mon, Jan 8, 2018 at 10:08 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
    > From: Tim Chen <tim.c.chen@linux.intel.com>
    >
    > Ensure an IBPB (Indirect branch prediction barrier) before every VCPU
    > switch.
    >
    > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
    > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    > ---
    > arch/x86/kvm/vmx.c | 9 +++++++++
    > 1 file changed, 9 insertions(+)
    >
    > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
    > index d00bcad7336e..bf127c570675 100644
    > --- a/arch/x86/kvm/vmx.c
    > +++ b/arch/x86/kvm/vmx.c
    > @@ -2375,6 +2375,8 @@ static void vmx_vcpu_load(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);
    > + if (have_spec_ctrl)
    > + wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
    > }
    >
    > if (!already_loaded) {
    > @@ -4029,6 +4031,13 @@ static void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
    > free_vmcs(loaded_vmcs->vmcs);
    > loaded_vmcs->vmcs = NULL;
    > WARN_ON(loaded_vmcs->shadow_vmcs != NULL);
    > +
    > + /*
    > + * The VMCS could be recycled, causing a false negative in
    > + * vmx_vcpu_load; block speculative execution.
    > + */
    > + if (have_spec_ctrl)
    > + wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
    > }
    >
    > static void vmx_nested_free_vmcs02(struct vcpu_vmx *vmx)
    > --
    > 1.8.3.1
    >
    >

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