[PATCH] KVM: Improvements for task switching

[Bernhard Kohl <bernhard.kohl@nsn.com>]

NSN's proprietary OS DMX sometimes does task switches.
To get it running in KVM the following changes were necessary:
Interrupt injection only with interrupt flag set.
Linking the tss->prev_task_link to itself removed.
Task linking is required for CALL and GATE.
Do not call skip_emulated_instruction() for GATE.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
---
 arch/x86/kvm/vmx.c |    3 ++-
 arch/x86/kvm/x86.c |   19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 5cf28df..eca57a3 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3357,7 +3357,8 @@ static void vmx_intr_assist(struct kvm_vcpu *vcpu)
 			enable_irq_window(vcpu);
 	}
 	if (vcpu->arch.interrupt.pending) {
-		vmx_inject_irq(vcpu, vcpu->arch.interrupt.nr);
+		if (vcpu->arch.interrupt_window_open)
+			vmx_inject_irq(vcpu, vcpu->arch.interrupt.nr);
 		if (kvm_cpu_has_interrupt(vcpu))
 			enable_irq_window(vcpu);
 	}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b556b6a..9052058 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3683,7 +3683,7 @@ static void save_state_to_tss32(struct kvm_vcpu *vcpu,
 	tss->fs = get_segment_selector(vcpu, VCPU_SREG_FS);
 	tss->gs = get_segment_selector(vcpu, VCPU_SREG_GS);
 	tss->ldt_selector = get_segment_selector(vcpu, VCPU_SREG_LDTR);
-	tss->prev_task_link = get_segment_selector(vcpu, VCPU_SREG_TR);
+	tss->prev_task_link = 0;
 }
 
 static int load_state_from_tss32(struct kvm_vcpu *vcpu,
@@ -3810,6 +3810,7 @@ out:
 
 static int kvm_task_switch_32(struct kvm_vcpu *vcpu, u16 tss_selector,
 		       u32 old_tss_base,
+		       u16 old_tss_selector, int reason,
 		       struct desc_struct *nseg_desc)
 {
 	struct tss_segment_32 tss_segment_32;
@@ -3829,6 +3830,18 @@ static int kvm_task_switch_32(struct kvm_vcpu *vcpu, u16
tss_selector,
 			   &tss_segment_32, sizeof tss_segment_32))
 		goto out;
 
+	/*
+	 * SDM 3: table 6-2
+	 * Task linking required for CALL and GATE.
+	 */
+	if (reason == TASK_SWITCH_CALL || reason == TASK_SWITCH_GATE)
+	{
+		tss_segment_32.prev_task_link = old_tss_selector;
+		kvm_write_guest(vcpu->kvm, get_tss_base_addr(vcpu, nseg_desc),
+				&tss_segment_32, sizeof(struct tss_segment_32));
+
+	}
+
 	if (load_state_from_tss32(vcpu, &tss_segment_32))
 		goto out;
 
@@ -3882,10 +3895,12 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16
tss_selector, int reason)
 		kvm_x86_ops->set_rflags(vcpu, eflags & ~X86_EFLAGS_NT);
 	}
 
-	kvm_x86_ops->skip_emulated_instruction(vcpu);
+	if (reason != TASK_SWITCH_GATE)
+		kvm_x86_ops->skip_emulated_instruction(vcpu);
 
 	if (nseg_desc.type & 8)
 		ret = kvm_task_switch_32(vcpu, tss_selector, old_tss_base,
+ 					 old_tss_sel, reason,
 					 &nseg_desc);
 	else
 		ret = kvm_task_switch_16(vcpu, tss_selector, old_tss_base,
-- 
1.6.0.6