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    Subject[PATCH] KVM: x86: fix root cause for missed hardware breakpoints
    Date
    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.

    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@vger.kernel.org # 4.1+
    Fixes: 172b2386ed16a9143d9a456aae5ec87275c61489
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    ---
    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)
    * 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;
    }

    /*
    --
    1.8.3.1
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      Last update: 2016-02-26 13:21    [W:3.502 / U:0.008 seconds]
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