Patch "KVM: x86: fix root cause for missed hardware breakpoints" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

KVM: x86: fix root cause for missed hardware breakpoints

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-x86-fix-root-cause-for-missed-hardware-breakpoints.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paolo Bonzini <[email protected]>
Date: Fri, 26 Feb 2016 12:28:40 +0100
Subject: KVM: x86: fix root cause for missed hardware breakpoints

From: Paolo Bonzini <[email protected]>

commit 70e4da7a8ff62f2775337b705f45c804bb450454 upstream.

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.

---
 arch/x86/kvm/x86.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

@@ -2736,7 +2736,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu
 	}
 
 	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)

@@ -6545,12 +6544,12 @@ static int vcpu_enter_guest(struct kvm_v
 	 * 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;
 	}
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kvm-x86-update-tsc-multiplier-on-change.patch queue-4.4/kvm-x86-fix-root-cause-for-missed-hardware-breakpoints.patch queue-4.4/mips-kvm-fix-ioctl-error-handling.patch