Re: [PATCH] KVM: x86: fix missed hardware breakpoints

From: Xiao Guangrong
Date: Fri Feb 26 2016 - 06:42:46 EST

On 02/26/2016 07:28 PM, Nadav Amit wrote:
Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx> wrote:

On 02/19/2016 06:56 PM, Paolo Bonzini wrote:
Sometimes when setting a breakpoint a process doesn't stop on it.
This is because the debug registers are not loaded correctly on
VCPU load.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4244c2baf57d..f4891f2ece23 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2752,6 +2752,7 @@ 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;

Er, i do not understand how it works. The BP is enabled in this test case so
the debug registers are always reloaded before entering guest as
KVM_DEBUGREG_BP_ENABLED bit is always set on switch_db_regs. What did i miss?

Note that KVM_DEBUGREG_BP_ENABLED does not have to be set, since
kvm_update_dr7() is not called once the guest has KVM_DEBUGREG_WONT_EXIT.

Ah, yes. So that we will enter guest without reloading debug registers under this
case. Is it safe? What about if gdb/perf attached to QEMU and reset the debug0ïï
(in this case, the vcpu is interrupted by IPI and it is not rescheduled)ï