KVM: x86: fix root cause for missed hardware breakpoints

Commit 172b2386ed16 ("KVM: x86: fix missed hardware breakpoints",
2016-02-10) worked around a case where the debug registers are not loaded
correctly on preemption and on the first entry to KVM_RUN.

However, Xiao Guangrong pointed out that the root cause must be that
KVM_DEBUGREG_BP_ENABLED is not being set correctly.  This can indeed
happen due to the lazy debug exit mechanism, which does not call
kvm_update_dr7.  Fix it by replacing the existing loop (more or less
equivalent to kvm_update_dr0123) with calls to all the kvm_update_dr*
functions.

Cc: stable@vger.kernel.org   # 4.1+
Fixes: 172b2386ed16a9143d9a456aae5ec87275c61489
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 5 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f4891f2ece23..eaf6ee8c28b8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2752,7 +2752,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
         }
 
         kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
-        vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
@@ -6619,12 +6618,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
          * KVM_DEBUGREG_WONT_EXIT again.
          */
         if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
-                int i;
-
                 WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);
                 kvm_x86_ops->sync_dirty_debug_regs(vcpu);
-                for (i = 0; i < KVM_NR_DB_REGS; i++)
-                        vcpu->arch.eff_db[i] = vcpu->arch.db[i];
+                kvm_update_dr0123(vcpu);
+                kvm_update_dr6(vcpu);
+                kvm_update_dr7(vcpu);
+                vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
         }
 
         /*