Re: [PATCH] KVM: x86: fix root cause for missed hardware breakpoints

From: Xiao Guangrong
Date: Fri Feb 26 2016 - 07:01:32 EST

On 02/26/2016 07:46 PM, Paolo Bonzini wrote:
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*

The original patch is good enough for stable releases before 4.1, since
it does not have any dependencies such as commit ae561edeb421 ("KVM:
x86: DR0-DR3 are not clear on reset", 2015-04-02).

Cc: stable@xxxxxxxxxxxxxxx # 4.1+
Fixes: 172b2386ed16a9143d9a456aae5ec87275c61489
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
arch/x86/kvm/x86.c | 9 ++++-----
1 file 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)
if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
- int i;
WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);
- 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;

It is good to me now. It also can fix the case that the debug registers are
changed by the debug-process(gdb/perf) in the context of the vcpu as i
described in another thread.

Reviewed-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>