[PATCH] KVM: Improve interrupt response

The current interrupt injection mechanism might delay an interrupt under
the following circumstances:

 - if injection fails because the guest is not interruptible (rflags.IF clear,
   or after a 'mov ss' or 'sti' instruction).  Userspace can check rflags,
   but the other cases or not testable under the current API.
 - if injection fails because of a fault during delivery.  This probably
   never happens under normal guests.
 - if injection fails due to a physical interrupt causing a vmexit so that
   it can be handled by the host.

In all cases the guest proceeds without processing the interrupt, reducing
the interactive feel and interrupt throughput of the guest.

This patch fixes the situation by allowing userspace to request an exit
when the 'interrupt window' opens, so that it can re-inject the interrupt
at the right time.  Guest interactivity is very visibly improved.

Signed-off-by: Dor Laor <dor.laor@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 72 insertions, 16 deletions

diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
index 2d204fd45972..c55635ddf426 100644
--- a/drivers/kvm/vmx.c
+++ b/drivers/kvm/vmx.c
@@ -263,6 +263,7 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
         if (interruptibility & 3)
                 vmcs_write32(GUEST_INTERRUPTIBILITY_INFO,
                              interruptibility & ~3);
+        vcpu->interrupt_window_open = 1;
 }
 
 static void vmx_inject_gp(struct kvm_vcpu *vcpu, unsigned error_code)
@@ -1214,21 +1215,34 @@ static void kvm_do_inject_irq(struct kvm_vcpu *vcpu)
                         irq | INTR_TYPE_EXT_INTR | INTR_INFO_VALID_MASK);
 }
 
-static void kvm_try_inject_irq(struct kvm_vcpu *vcpu)
+
+static void do_interrupt_requests(struct kvm_vcpu *vcpu,
+                                       struct kvm_run *kvm_run)
 {
-        if ((vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF)
-            && (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & 3) == 0)
+        u32 cpu_based_vm_exec_control;
+
+        vcpu->interrupt_window_open =
+                ((vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF) &&
+                 (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & 3) == 0);
+
+        if (vcpu->interrupt_window_open &&
+            vcpu->irq_summary &&
+            !(vmcs_read32(VM_ENTRY_INTR_INFO_FIELD) & INTR_INFO_VALID_MASK))
                 /*
-                 * Interrupts enabled, and not blocked by sti or mov ss. Good.
+                 * If interrupts enabled, and not blocked by sti or mov ss. Good.
                  */
                 kvm_do_inject_irq(vcpu);
-        else
+
+        cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
+        if (!vcpu->interrupt_window_open &&
+            (vcpu->irq_summary || kvm_run->request_interrupt_window))
                 /*
                  * Interrupts blocked.  Wait for unblock.
                  */
-                vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
-                             vmcs_read32(CPU_BASED_VM_EXEC_CONTROL)
-                             | CPU_BASED_VIRTUAL_INTR_PENDING);
+                cpu_based_vm_exec_control |= CPU_BASED_VIRTUAL_INTR_PENDING;
+        else
+                cpu_based_vm_exec_control &= ~CPU_BASED_VIRTUAL_INTR_PENDING;
+        vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
 }
 
 static void kvm_guest_debug_pre(struct kvm_vcpu *vcpu)
@@ -1565,23 +1579,41 @@ static int handle_wrmsr(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
         return 1;
 }
 
+static void post_kvm_run_save(struct kvm_vcpu *vcpu,
+                              struct kvm_run *kvm_run)
+{
+        kvm_run->if_flag = (vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF) != 0;
+        kvm_run->cr8 = vcpu->cr8;
+        kvm_run->apic_base = vcpu->apic_base;
+        kvm_run->ready_for_interrupt_injection = (vcpu->interrupt_window_open &&
+                                                  vcpu->irq_summary == 0);
+}
+
 static int handle_interrupt_window(struct kvm_vcpu *vcpu,
                                    struct kvm_run *kvm_run)
 {
-        /* Turn off interrupt window reporting. */
-        vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
-                     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL)
-                     & ~CPU_BASED_VIRTUAL_INTR_PENDING);
+        /*
+         * If the user space waits to inject interrupts, exit as soon as
+         * possible
+         */
+        if (kvm_run->request_interrupt_window &&
+            !vcpu->irq_summary &&
+            (vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF)) {
+                kvm_run->exit_reason = KVM_EXIT_IRQ_WINDOW_OPEN;
+                ++kvm_stat.irq_window_exits;
+                return 0;
+        }
         return 1;
 }
 
 static int handle_halt(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 {
         skip_emulated_instruction(vcpu);
-        if (vcpu->irq_summary && (vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF))
+        if (vcpu->irq_summary)
                 return 1;
 
         kvm_run->exit_reason = KVM_EXIT_HLT;
+        ++kvm_stat.halt_exits;
         return 0;
 }
 
@@ -1632,6 +1664,21 @@ static int kvm_handle_exit(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
         return 0;
 }
 
+/*
+ * Check if userspace requested an interrupt window, and that the
+ * interrupt window is open.
+ *
+ * No need to exit to userspace if we already have an interrupt queued.
+ */
+static int dm_request_for_irq_injection(struct kvm_vcpu *vcpu,
+                                          struct kvm_run *kvm_run)
+{
+        return (!vcpu->irq_summary &&
+                kvm_run->request_interrupt_window &&
+                vcpu->interrupt_window_open &&
+                (vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF));
+}
+
 static int vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 {
         u8 fail;
@@ -1663,9 +1710,7 @@ again:
         vmcs_writel(HOST_GS_BASE, segment_base(gs_sel));
 #endif
 
-        if (vcpu->irq_summary &&
-            !(vmcs_read32(VM_ENTRY_INTR_INFO_FIELD) & INTR_INFO_VALID_MASK))
-                kvm_try_inject_irq(vcpu);
+        do_interrupt_requests(vcpu, kvm_run);
 
         if (vcpu->guest_debug.enabled)
                 kvm_guest_debug_pre(vcpu);
@@ -1802,6 +1847,7 @@ again:
 
         fx_save(vcpu->guest_fx_image);
         fx_restore(vcpu->host_fx_image);
+        vcpu->interrupt_window_open = (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & 3) == 0;
 
 #ifndef CONFIG_X86_64
         asm ("mov %0, %%ds; mov %0, %%es" : : "r"(__USER_DS));
@@ -1834,12 +1880,22 @@ again:
                         /* Give scheduler a change to reschedule. */
                         if (signal_pending(current)) {
                                 ++kvm_stat.signal_exits;
+                                post_kvm_run_save(vcpu, kvm_run);
+                                return -EINTR;
+                        }
+
+                        if (dm_request_for_irq_injection(vcpu, kvm_run)) {
+                                ++kvm_stat.request_irq_exits;
+                                post_kvm_run_save(vcpu, kvm_run);
                                 return -EINTR;
                         }
+
                         kvm_resched(vcpu);
                         goto again;
                 }
         }
+
+        post_kvm_run_save(vcpu, kvm_run);
         return 0;
 }