linux-kernel - Re: [PATCH 03/11] Handle asynchronous page fault in a PV guest.

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Date:	Mon, 02 Nov 2009 14:38:30 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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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