[PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
registers") allows the guest accessing to DRs without exiting when
KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
on entry to the guest---including DR6 that was not synced before the commit.

But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary
and just leads to a more case which leaks stale DR6 to the host which has
to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().

We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT,
so that we can fine-grain control the cases when we need to reset it
which is done in later patch.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kvm/x86.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ad47a09ce307..d2aa49722064 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		set_debugreg(vcpu->arch.eff_db[1], 1);
 		set_debugreg(vcpu->arch.eff_db[2], 2);
 		set_debugreg(vcpu->arch.eff_db[3], 3);
-		set_debugreg(vcpu->arch.dr6, 6);
+		/* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
+		if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
+			set_debugreg(vcpu->arch.dr6, 6);
 	} else if (unlikely(hw_breakpoint_active())) {
 		set_debugreg(0, 7);
 	}
-- 
2.19.1.6.gb485710b