[Devel] [PATCH vz7.16 2/2] KVM: x86: fix root cause for missed hardware breakpoints

[Roman Kagan]

From: Paolo Bonzini <pbonzini at redhat.com>

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 at vger.kernel.org   # 4.1+
Fixes: 172b2386ed16a9143d9a456aae5ec87275c61489
Reviewed-by: Xiao Guangrong <guangrong.xiao at linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
(cherry picked from commit 70e4da7a8ff62f2775337b705f45c804bb450454)
Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
---
 arch/x86/kvm/x86.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c906f883..6461a83 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6512,12 +6512,12 @@ 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;
 	}
 
 	/*
-- 
2.7.4