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    SubjectRe: [PATCH 03/11] Handle asynchronous page fault in a PV guest.
    On 11/01/2009 01:56 PM, Gleb Natapov wrote:
    > 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.
    >
    > diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
    > index 90708b7..61e2aa3 100644
    > --- a/arch/x86/include/asm/kvm_para.h
    > +++ b/arch/x86/include/asm/kvm_para.h
    > @@ -52,6 +52,9 @@ struct kvm_mmu_op_release_pt {
    >
    > #define KVM_PV_SHM_FEATURES_ASYNC_PF (1<< 0)
    >
    > +#define KVM_PV_REASON_PAGE_NP 1
    > +#define KVM_PV_REASON_PAGE_READY 2
    >

    _NOT_PRESENT would improve readability.

    > +static void apf_task_wait(struct task_struct *tsk, u64 token)
    > {
    > + u64 key = hash_64(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;
    >

    Don't you need locking here? At least for the implied memory barriers.

    > + schedule();
    > + }
    > + finish_wait(&n.wq,&wait);
    > +
    > + return;
    > +}
    > +
    > +static void apf_task_wake(u64 token)
    > +{
    > + u64 key = hash_64(token, KVM_TASK_SLEEP_HASHBITS);
    > + struct kvm_task_sleep_head *b =&async_pf_sleepers[key];
    > + struct kvm_task_sleep_node *n;
    > +
    > + spin_lock(&b->lock);
    > + n = _find_apf_task(b, token);
    > + if (!n) {
    > + /* PF was not yet handled. Add dummy entry for the token */
    > + n = kmalloc(sizeof(*n), GFP_ATOMIC);
    > + if (!n) {
    > + printk(KERN_EMERG"async PF can't allocate memory\n");
    >

    Worrying. We could have an emergency pool of one node per cpu, and
    disable apf if we use it until it's returned. But that's a lot of
    complexity for an edge case, so a simpler solution would be welcome.

    > +int kvm_handle_pf(struct pt_regs *regs, unsigned long error_code)
    > +{
    > + u64 reason, token;
    > struct kvm_vcpu_pv_shm *pv_shm =
    > per_cpu(kvm_vcpu_pv_shm, smp_processor_id());
    >
    > if (!pv_shm)
    > - return;
    > + return 0;
    > +
    > + reason = pv_shm->reason;
    > + pv_shm->reason = 0;
    > +
    > + token = pv_shm->param;
    > +
    > + switch (reason) {
    > + default:
    > + return 0;
    > + case KVM_PV_REASON_PAGE_NP:
    > + /* real page is missing. */
    > + apf_task_wait(current, token);
    > + break;
    > + case KVM_PV_REASON_PAGE_READY:
    > + apf_task_wake(token);
    > + break;
    > + }
    >

    Ah, reason is not a bitmask but an enumerator. __u32 is more friendly
    to i386 in that case.

    Much of the code here is arch independent and would work well on non-x86
    kvm ports. But we can always lay the burden of moving it on the non-x86
    maintainers.

    --
    error compiling committee.c: too many arguments to function



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