KVM: X86: Set host DR6 only on VMX and for KVM_DEBUGREG_WONT_EXIT

Commit c77fb5fe ("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().

Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters
on VMX because SVM always uses the DR6 value from the VMCB.  So move this
line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT.
Reported-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		vmx->loaded_vmcs->host_state.cr4 = cr4;
 	}
 
+	/* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
+	if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
+		set_debugreg(vcpu->arch.dr6, 6);
+
 	/* When single-stepping over STI and MOV SS, we must clear the
 	 * corresponding interruptibility bits in the guest state. Otherwise
 	 * vmentry fails as it then expects bit 14 (BS) in pending debug

arch/x86/kvm/x86.c

@@ -9602,7 +9602,6 @@ 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);
 	} else if (unlikely(hw_breakpoint_active())) {
 		set_debugreg(0, 7);
 	}