KVM: SVM: Don't set current_vmcb->cpu when switching vmcb

Do not update the new vmcb's last-run cpu when switching to a different
vmcb.  If the vCPU is migrated between its last run and a vmcb switch,
e.g. for nested VM-Exit, then setting the cpu without marking the vmcb
dirty will lead to KVM running the vCPU on a different physical cpu with
stale clean bit settings.

                          vcpu->cpu    current_vmcb->cpu    hardware
  pre_svm_run()           cpu0         cpu0                 cpu0,clean
  kvm_arch_vcpu_load()    cpu1         cpu0                 cpu0,clean
  svm_switch_vmcb()       cpu1         cpu1                 cpu0,clean
  pre_svm_run()           cpu1         cpu1                 kaboom

Simply delete the offending code; unlike VMX, which needs to update the
cpu at switch time due to the need to do VMPTRLD, SVM only cares about
which cpu last ran the vCPU.

Fixes: af18fa775d07 ("KVM: nSVM: Track the physical cpu of the vmcb vmrun through the vmcb")
Cc: Cathy Avery <cavery@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406171811.4043363-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4fc1f16505db18a3d7dafb069cb808591c0f5106..04ef49d3862d37f15bd2b9935f047f3c015fdb27 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1311,14 +1311,6 @@ void svm_switch_vmcb(struct vcpu_svm *svm, struct kvm_vmcb_info *target_vmcb)
        svm->current_vmcb = target_vmcb;
        svm->vmcb = target_vmcb->ptr;
        svm->vmcb_pa = target_vmcb->pa;
-
-       /*
-       * Track the physical CPU the target_vmcb is running on
-       * in order to mark the VMCB dirty if the cpu changes at
-       * its next vmrun.
-       */
-
-       svm->current_vmcb->cpu = svm->vcpu.cpu;
 }
 
 static int svm_create_vcpu(struct kvm_vcpu *vcpu)