Messages in this thread | | | Date | Mon, 02 Nov 2009 14:38:30 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH 03/11] Handle asynchronous page fault in a PV guest. |
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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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