KVM paravirt: Handle async PF in non preemptable context

If async page fault is received by idle task or when preemp_count is
not zero guest cannot reschedule, so do sti; hlt and wait for page to be
ready. vcpu can still process interrupts while it waits for the page to
be ready.

Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 34 insertions, 6 deletions

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d5640634fef6..47ea93e6b0d8 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -37,6 +37,7 @@
 #include <asm/cpu.h>
 #include <asm/traps.h>
 #include <asm/desc.h>
+#include <asm/tlbflush.h>
 
 #define MMU_QUEUE_SIZE 1024
 
@@ -78,6 +79,8 @@ struct kvm_task_sleep_node {
         wait_queue_head_t wq;
         u32 token;
         int cpu;
+        bool halted;
+        struct mm_struct *mm;
 };
 
 static struct kvm_task_sleep_head {
@@ -106,6 +109,11 @@ void kvm_async_pf_task_wait(u32 token)
         struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
         struct kvm_task_sleep_node n, *e;
         DEFINE_WAIT(wait);
+        int cpu, idle;
+
+        cpu = get_cpu();
+        idle = idle_cpu(cpu);
+        put_cpu();
 
         spin_lock(&b->lock);
         e = _find_apf_task(b, token);
@@ -119,19 +127,33 @@ void kvm_async_pf_task_wait(u32 token)
 
         n.token = token;
         n.cpu = smp_processor_id();
+        n.mm = current->active_mm;
+        n.halted = idle || preempt_count() > 1;
+        atomic_inc(&n.mm->mm_count);
         init_waitqueue_head(&n.wq);
         hlist_add_head(&n.link, &b->list);
         spin_unlock(&b->lock);
 
         for (;;) {
-                prepare_to_wait(&n.wq, &wait, TASK_UNINTERRUPTIBLE);
+                if (!n.halted)
+                        prepare_to_wait(&n.wq, &wait, TASK_UNINTERRUPTIBLE);
                 if (hlist_unhashed(&n.link))
                         break;
-                local_irq_enable();
-                schedule();
-                local_irq_disable();
+
+                if (!n.halted) {
+                        local_irq_enable();
+                        schedule();
+                        local_irq_disable();
+                } else {
+                        /*
+                         * We cannot reschedule. So halt.
+                         */
+                        native_safe_halt();
+                        local_irq_disable();
+                }
         }
-        finish_wait(&n.wq, &wait);
+        if (!n.halted)
+                finish_wait(&n.wq, &wait);
 
         return;
 }
@@ -140,7 +162,12 @@ EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
 static void apf_task_wake_one(struct kvm_task_sleep_node *n)
 {
         hlist_del_init(&n->link);
-        if (waitqueue_active(&n->wq))
+        if (!n->mm)
+                return;
+        mmdrop(n->mm);
+        if (n->halted)
+                smp_send_reschedule(n->cpu);
+        else if (waitqueue_active(&n->wq))
                 wake_up(&n->wq);
 }
 
@@ -193,6 +220,7 @@ again:
                 }
                 n->token = token;
                 n->cpu = smp_processor_id();
+                n->mm = NULL;
                 init_waitqueue_head(&n->wq);
                 hlist_add_head(&n->link, &b->list);
         } else