KVM: Fix race between timer migration and vcpu migration

A guest vcpu instance can be scheduled to a different physical CPU
between the test for KVM_REQ_MIGRATE_TIMER and local_irq_disable().

If that happens, the timer will only be migrated to the current pCPU on
the next exit, meaning that guest LAPIC timer event can be delayed until
a host interrupt is triggered.

Fix it by cancelling guest entry if any vcpu request is pending.  This
has the side effect of nicely consolidating vcpu->requests checks.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 00acf13..b90744a 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2759,6 +2759,8 @@ again:
        if (vcpu->requests) {
                if (test_and_clear_bit(KVM_REQ_MIGRATE_TIMER, &vcpu->requests))
                        __kvm_migrate_timers(vcpu);
+               if (test_and_clear_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests))
+                       kvm_x86_ops->tlb_flush(vcpu);
                if (test_and_clear_bit(KVM_REQ_REPORT_TPR_ACCESS,
                                       &vcpu->requests)) {
                        kvm_run->exit_reason = KVM_EXIT_TPR_ACCESS;
@@ -2781,21 +2783,13 @@ again:
 
        local_irq_disable();
 
-       if (need_resched()) {
+       if (vcpu->requests || need_resched()) {
                local_irq_enable();
                preempt_enable();
                r = 1;
                goto out;
        }
 
-       if (vcpu->requests)
-               if (test_bit(KVM_REQ_MMU_RELOAD, &vcpu->requests)) {
-                       local_irq_enable();
-                       preempt_enable();
-                       r = 1;
-                       goto out;
-               }
-
        if (signal_pending(current)) {
                local_irq_enable();
                preempt_enable();
@@ -2825,9 +2819,6 @@ again:
 
        kvm_guest_enter();
 
-       if (vcpu->requests)
-               if (test_and_clear_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests))
-                       kvm_x86_ops->tlb_flush(vcpu);
 
        KVMTRACE_0D(VMENTRY, vcpu, entryexit);
        kvm_x86_ops->run(vcpu, kvm_run);