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

[Xiao Guangrong]

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*
> 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@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)
>   	 * 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;
>   	}

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>