KVM: Handle async PF in a guest.

When async PF capability is detected hook up special page fault handler
that will handle async page fault events and bypass other page faults to
regular page fault handler. Also add async PF handling to nested SVM
emulation. Async PF always generates exit to L1 where vcpu thread will
be scheduled out until page is available.

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

3 files changed, 194 insertions, 0 deletions

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 591e601..c8b4efa 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1406,6 +1406,16 @@ ENTRY(general_protection)
 	CFI_ENDPROC
 END(general_protection)
 
+#ifdef CONFIG_KVM_GUEST
+ENTRY(async_page_fault)
+	RING0_EC_FRAME
+	pushl $do_async_page_fault
+	CFI_ADJUST_CFA_OFFSET 4
+	jmp error_code
+	CFI_ENDPROC
+END(apf_page_fault)
+#endif
+
 /*
  * End of kprobes section
  */
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e3ba417..bb3f6e9 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1319,6 +1319,9 @@ errorentry xen_stack_segment do_stack_segment
 #endif
 errorentry general_protection do_general_protection
 errorentry page_fault do_page_fault
+#ifdef CONFIG_KVM_GUEST
+errorentry async_page_fault do_async_page_fault
+#endif
 #ifdef CONFIG_X86_MCE
 paranoidzeroentry machine_check *machine_check_vector(%rip)
 #endif
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 032d03b..d564063 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -29,8 +29,14 @@
 #include <linux/hardirq.h>
 #include <linux/notifier.h>
 #include <linux/reboot.h>
+#include <linux/hash.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/kprobes.h>
 #include <asm/timer.h>
 #include <asm/cpu.h>
+#include <asm/traps.h>
+#include <asm/desc.h>
 
 #define MMU_QUEUE_SIZE 1024
 
@@ -64,6 +70,168 @@ static void kvm_io_delay(void)
 {
 }
 
+#define KVM_TASK_SLEEP_HASHBITS 8
+#define KVM_TASK_SLEEP_HASHSIZE (1<<KVM_TASK_SLEEP_HASHBITS)
+
+struct kvm_task_sleep_node {
+	struct hlist_node link;
+	wait_queue_head_t wq;
+	u32 token;
+	int cpu;
+};
+
+static struct kvm_task_sleep_head {
+	spinlock_t lock;
+	struct hlist_head list;
+} async_pf_sleepers[KVM_TASK_SLEEP_HASHSIZE];
+
+static struct kvm_task_sleep_node *_find_apf_task(struct kvm_task_sleep_head *b,
+						  u32 token)
+{
+	struct hlist_node *p;
+
+	hlist_for_each(p, &b->list) {
+		struct kvm_task_sleep_node *n =
+			hlist_entry(p, typeof(*n), link);
+		if (n->token == token)
+			return n;
+	}
+
+	return NULL;
+}
+
+void kvm_async_pf_task_wait(u32 token)
+{
+	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
+	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
+	struct kvm_task_sleep_node n, *e;
+	DEFINE_WAIT(wait);
+
+	spin_lock(&b->lock);
+	e = _find_apf_task(b, token);
+	if (e) {
+		/* dummy entry exist -> wake up was delivered ahead of PF */
+		hlist_del(&e->link);
+		kfree(e);
+		spin_unlock(&b->lock);
+		return;
+	}
+
+	n.token = token;
+	n.cpu = smp_processor_id();
+	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 (hlist_unhashed(&n.link))
+			break;
+		local_irq_enable();
+		schedule();
+		local_irq_disable();
+	}
+	finish_wait(&n.wq, &wait);
+
+	return;
+}
+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))
+		wake_up(&n->wq);
+}
+
+static void apf_task_wake_all(void)
+{
+	int i;
+
+	for (i = 0; i < KVM_TASK_SLEEP_HASHSIZE; i++) {
+		struct hlist_node *p, *next;
+		struct kvm_task_sleep_head *b = &async_pf_sleepers[i];
+		spin_lock(&b->lock);
+		hlist_for_each_safe(p, next, &b->list) {
+			struct kvm_task_sleep_node *n =
+				hlist_entry(p, typeof(*n), link);
+			if (n->cpu == smp_processor_id())
+				apf_task_wake_one(n);
+		}
+		spin_unlock(&b->lock);
+	}
+}
+
+void kvm_async_pf_task_wake(u32 token)
+{
+	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
+	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
+	struct kvm_task_sleep_node *n;
+
+	if (token == ~0) {
+		apf_task_wake_all();
+		return;
+	}
+
+again:
+	spin_lock(&b->lock);
+	n = _find_apf_task(b, token);
+	if (!n) {
+		/*
+		 * async PF was not yet handled.
+		 * Add dummy entry for the token.
+		 */
+		n = kmalloc(sizeof(*n), GFP_ATOMIC);
+		if (!n) {
+			/*
+			 * Allocation failed! Busy wait while other cpu
+			 * handles async PF.
+			 */
+			spin_unlock(&b->lock);
+			cpu_relax();
+			goto again;
+		}
+		n->token = token;
+		n->cpu = smp_processor_id();
+		init_waitqueue_head(&n->wq);
+		hlist_add_head(&n->link, &b->list);
+	} else
+		apf_task_wake_one(n);
+	spin_unlock(&b->lock);
+	return;
+}
+EXPORT_SYMBOL_GPL(kvm_async_pf_task_wake);
+
+u32 kvm_read_and_reset_pf_reason(void)
+{
+	u32 reason = 0;
+
+	if (__get_cpu_var(apf_reason).enabled) {
+		reason = __get_cpu_var(apf_reason).reason;
+		__get_cpu_var(apf_reason).reason = 0;
+	}
+
+	return reason;
+}
+EXPORT_SYMBOL_GPL(kvm_read_and_reset_pf_reason);
+
+dotraplinkage void __kprobes
+do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
+{
+	switch (kvm_read_and_reset_pf_reason()) {
+	default:
+		do_page_fault(regs, error_code);
+		break;
+	case KVM_PV_REASON_PAGE_NOT_PRESENT:
+		/* page is swapped out by the host. */
+		kvm_async_pf_task_wait((u32)read_cr2());
+		break;
+	case KVM_PV_REASON_PAGE_READY:
+		kvm_async_pf_task_wake((u32)read_cr2());
+		break;
+	}
+}
+
 static void kvm_mmu_op(void *buffer, unsigned len)
 {
 	int r;
@@ -300,6 +468,7 @@ static void kvm_guest_cpu_online(void *dummy)
 static void kvm_guest_cpu_offline(void *dummy)
 {
 	kvm_pv_disable_apf(NULL);
+	apf_task_wake_all();
 }
 
 static int __cpuinit kvm_cpu_notify(struct notifier_block *self,
@@ -327,13 +496,25 @@ static struct notifier_block __cpuinitdata kvm_cpu_notifier = {
 };
 #endif
 
+static void __init kvm_apf_trap_init(void)
+{
+	set_intr_gate(14, &async_page_fault);
+}
+
 void __init kvm_guest_init(void)
 {
+	int i;
+
 	if (!kvm_para_available())
 		return;
 
 	paravirt_ops_setup();
 	register_reboot_notifier(&kvm_pv_reboot_nb);
+	for (i = 0; i < KVM_TASK_SLEEP_HASHSIZE; i++)
+		spin_lock_init(&async_pf_sleepers[i].lock);
+	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF))
+		x86_init.irqs.trap_init = kvm_apf_trap_init;
+
 #ifdef CONFIG_SMP
 	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
 	register_cpu_notifier(&kvm_cpu_notifier);