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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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