[PATCH v2 05/12] Handle asynchronous page fault in a PV guest.

Gleb Natapov
2009-11-23T14:06:00Z

Asynchronous page fault notifies vcpu that page it is trying to access
is swapped out by a host. In response guest puts a task that caused the
fault to sleep until page is swapped in again. When missing page is
brought back into the memory guest is notified and task resumes execution.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
---
arch/x86/include/asm/kvm_para.h | 3 +
arch/x86/kernel/kvm.c | 132 +++++++++++++++++++++++++++++++++++++++
2 files changed, 135 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index d7d7079..79bb7f2 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -49,6 +49,9 @@ struct kvm_mmu_op_release_pt {
__u64 pt_phys;
};

+#define KVM_PV_REASON_PAGE_NOT_PRESENT 1
+#define KVM_PV_REASON_PAGE_READY 2
+
struct kvm_vcpu_pv_apf_data {
__u32 reason;
__u32 enabled;
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index fdd0b95..09444c9 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -29,6 +29,8 @@
#include <linux/hardirq.h>
#include <linux/notifier.h>
#include <linux/reboot.h>
+#include <linux/hash.h>
+#include <linux/sched.h>
#include <asm/timer.h>
#include <asm/cpu.h>

@@ -54,6 +56,130 @@ 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;
+};
+
+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,
+ u64 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;
+}
+
+static void apf_task_wait(struct task_struct *tsk, 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;
+ 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;
+ schedule();
+ }
+ finish_wait(&n.wq, &wait);
+
+ return;
+}
+
+static void apf_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;
+
+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 vcpu
+ * handles async PF.
+ */
+ spin_unlock(&b->lock);
+ cpu_relax();
+ goto again;
+ }
+ n->token = token;
+ hlist_add_head(&n->link, &b->list);
+ } else {
+ hlist_del_init(&n->link);
+ if (waitqueue_active(&n->wq))
+ wake_up(&n->wq);
+ }
+ spin_unlock(&b->lock);
+ return;
+}
+
+int kvm_handle_pf(struct pt_regs *regs, unsigned long error_code)
+{
+ u32 reason, token;
+
+ if (!per_cpu(apf_reason, smp_processor_id()).enabled)
+ return 0;
+
+ reason = per_cpu(apf_reason, smp_processor_id()).reason;
+ per_cpu(apf_reason, smp_processor_id()).reason = 0;
+
+ token = (u32)read_cr2();
+
+ switch (reason) {
+ default:
+ return 0;
+ case KVM_PV_REASON_PAGE_NOT_PRESENT:
+ /* page is swapped out by the host. */
+ apf_task_wait(current, token);
+ break;
+ case KVM_PV_REASON_PAGE_READY:
+ apf_task_wake(token);
+ break;
+ }
+
+ return 1;
+}
+
static void kvm_mmu_op(void *buffer, unsigned len)
{
int r;
@@ -207,6 +333,9 @@ static void __init paravirt_ops_setup(void)
if (kvm_para_has_feature(KVM_FEATURE_NOP_IO_DELAY))
pv_cpu_ops.io_delay = kvm_io_delay;

+ if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF))
+ pv_cpu_ops.handle_pf = kvm_handle_pf;
+
if (kvm_para_has_feature(KVM_FEATURE_MMU_OP)) {
pv_mmu_ops.set_pte = kvm_set_pte;
pv_mmu_ops.set_pte_at = kvm_set_pte_at;
@@ -270,11 +399,14 @@ static void __init kvm_smp_prepare_boot_cpu(void)

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);
#ifdef CONFIG_SMP
smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
#else
--
1.6.5