Date:	Mon, 22 Jun 2009 08:56:28 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, mtosatti@...hat.com,
	paulmck@...ux.vnet.ibm.com, markmc@...hat.com
Subject: Re: [KVM PATCH v8 3/3] KVM: add iosignalfd support

Michael S. Tsirkin wrote:
> On Mon, Jun 22, 2009 at 08:13:48AM -0400, Gregory Haskins wrote:
>   
>>>> + * notification when the memory has been touched.
>>>> + * --------------------------------------------------------------------
>>>> + */
>>>> +
>>>> +/*
>>>> + * Design note: We create one PIO/MMIO device (iosignalfd_group) which
>>>> + * aggregates  one or more iosignalfd_items.  Each item points to exactly one
>>>>         
>                    ^^                             ^^
>   
>>>> + * eventfd, and can be registered to trigger on any write to the group
>>>> + * (wildcard), or to a write of a specific value.  If more than one item is to
>>>>         
>                                                        ^^
>   
>>>> + * be supported, the addr/len ranges must all be identical in the group.  If a
>>>>         
>                                                                               ^^
>   
>>>> + * trigger value is to be supported on a particular item, the group range must
>>>> + * be exactly the width of the trigger.
>>>>     
>>>>         
>>> Some duplicate spaces in the text above, apparently at random places.
>>>
>>>   
>>>       
>> -ENOPARSE ;)
>>
>> Can you elaborate?
>>     
>
>
> Marked with ^^
>   
Heh...well, the first one ("aggregates   one") is just a plain typo. 
The others are just me showing my age, perhaps:

http://desktoppub.about.com/cs/typespacing/a/onetwospaces.htm

Whether right or wrong, I think I use two-spaces-after-a-period
everywhere.  I can fix these if they bother you, but I suspect just
about every comment I've written has them too. ;)

-Greg


>   
>>>> + */
>>>> +
>>>> +struct _iosignalfd_item {
>>>> +	struct list_head     list;
>>>> +	struct file         *file;
>>>> +	u64                  match;
>>>> +	struct rcu_head      rcu;
>>>> +	int                  wildcard:1;
>>>> +};
>>>> +
>>>> +struct _iosignalfd_group {
>>>> +	struct list_head     list;
>>>> +	u64                  addr;
>>>> +	size_t               length;
>>>> +	size_t               count;
>>>> +	struct list_head     items;
>>>> +	struct kvm_io_device dev;
>>>> +	struct rcu_head      rcu;
>>>> +};
>>>> +
>>>> +static inline struct _iosignalfd_group *
>>>> +to_group(struct kvm_io_device *dev)
>>>> +{
>>>> +	return container_of(dev, struct _iosignalfd_group, dev);
>>>> +}
>>>> +
>>>> +static void
>>>> +iosignalfd_item_free(struct _iosignalfd_item *item)
>>>> +{
>>>> +	fput(item->file);
>>>> +	kfree(item);
>>>> +}
>>>> +
>>>> +static void
>>>> +iosignalfd_item_deferred_free(struct rcu_head *rhp)
>>>> +{
>>>> +	struct _iosignalfd_item *item;
>>>> +
>>>> +	item = container_of(rhp, struct _iosignalfd_item, rcu);
>>>> +
>>>> +	iosignalfd_item_free(item);
>>>> +}
>>>> +
>>>> +static void
>>>> +iosignalfd_group_deferred_free(struct rcu_head *rhp)
>>>> +{
>>>> +	struct _iosignalfd_group *group;
>>>> +
>>>> +	group = container_of(rhp, struct _iosignalfd_group, rcu);
>>>> +
>>>> +	kfree(group);
>>>> +}
>>>> +
>>>> +static int
>>>> +iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
>>>> +			  int is_write)
>>>> +{
>>>> +	struct _iosignalfd_group *p = to_group(this);
>>>> +
>>>> +	return ((addr >= p->addr && (addr < p->addr + p->length)));
>>>> +}
>>>>     
>>>>         
>>> What does this test? len is ignored ...
>>>
>>>   
>>>       
>> Yeah, I was following precedent with other IO devices.  However, this
>> *is* sloppy, I agree.  Will fix.
>>
>>     
>>>> +
>>>> +static int
>>>>     
>>>>         
>>> This seems to be returning bool ...
>>>   
>>>       
>> Ack
>>     
>>>   
>>>       
>>>> +iosignalfd_is_match(struct _iosignalfd_group *group,
>>>> +		    struct _iosignalfd_item *item,
>>>> +		    const void *val,
>>>> +		    int len)
>>>> +{
>>>> +	u64 _val;
>>>> +
>>>> +	if (len != group->length)
>>>> +		/* mis-matched length is always a miss */
>>>> +		return false;
>>>>     
>>>>         
>>> Why is that? what if there's 8 byte write which covers
>>> a 4 byte group?
>>>   
>>>       
>> v7 and earlier used to allow that for wildcards, actually.   It of
>> course would never make sense to allow mis-matched writes for
>> non-wildcards, since the idea is to match the value exactly.  However,
>> the feedback I got from Avi was that we should make the wildcard vs
>> non-wildcard access symmetrical and ensure they both conform to the size.
>>     
>>>   
>>>       
>>>> +
>>>> +	if (item->wildcard)
>>>> +		/* wildcard is always a hit */
>>>> +		return true;
>>>> +
>>>> +	/* otherwise, we have to actually compare the data */
>>>> +
>>>> +	if (!IS_ALIGNED((unsigned long)val, len))
>>>> +		/* protect against this request causing a SIGBUS */
>>>> +		return false;
>>>>     
>>>>         
>>> Could you explain what this does please?
>>>   
>>>       
>> Sure:  item->match is a fixed u64 to represent all group->length
>> values.  So it might have a 1, 2, 4, or 8 byte value in it.  When I
>> write arrives, we need to cast the data-register (in this case
>> represented by (void*)val) into a u64 so the equality check (see [A],
>> below) can be done.  However, you can't cast an unaligned pointer, or it
>> will SIGBUS on many (most?) architectures.
>>     
>
> I mean guest access. Does it have to be aligned?
> You could memcpy the value...
>
>   
>>> I thought misaligned accesses are allowed.
>>>   
>>>       
>> If thats true, we are in trouble ;)
>>     
>
> I think it works at least on x86:
> http://en.wikipedia.org/wiki/Packed#x86_and_x86-64
>
>   
>>>   
>>>       
>>>> +
>>>> +	switch (len) {
>>>> +	case 1:
>>>> +		_val = *(u8 *)val;
>>>> +		break;
>>>> +	case 2:
>>>> +		_val = *(u16 *)val;
>>>> +		break;
>>>> +	case 4:
>>>> +		_val = *(u32 *)val;
>>>> +		break;
>>>> +	case 8:
>>>> +		_val = *(u64 *)val;
>>>> +		break;
>>>> +	default:
>>>> +		return false;
>>>> +	}
>>>>     
>>>>         
>>> So legal values for len are 1,2,4 and 8?
>>> Might be a good idea to document this.
>>>
>>>   
>>>       
>> Ack
>>
>>     
>>>> +
>>>> +	return _val == item->match;
>>>>     
>>>>         
>> [A]
>>
>>     
>>>> +}
>>>> +
>>>> +/*
>>>> + * MMIO/PIO writes trigger an event (if the data matches).
>>>> + *
>>>> + * This is invoked by the io_bus subsystem in response to an address match
>>>> + * against the group.  We must then walk the list of individual items to check
>>>> + * for a match and, if applicable, to send the appropriate signal. If the item
>>>> + * in question does not have a "match" pointer, it is considered a wildcard
>>>> + * and will always generate a signal.  There can be an arbitrary number
>>>> + * of distinct matches or wildcards per group.
>>>> + */
>>>> +static void
>>>> +iosignalfd_group_write(struct kvm_io_device *this, gpa_t addr, int len,
>>>> +		       const void *val)
>>>> +{
>>>> +	struct _iosignalfd_group *group = to_group(this);
>>>> +	struct _iosignalfd_item *item;
>>>> +
>>>> +	rcu_read_lock();
>>>> +
>>>> +	list_for_each_entry_rcu(item, &group->items, list) {
>>>> +		if (iosignalfd_is_match(group, item, val, len))
>>>> +			eventfd_signal(item->file, 1);
>>>> +	}
>>>> +
>>>> +	rcu_read_unlock();
>>>> +}
>>>> +
>>>> +/*
>>>> + * MMIO/PIO reads against the group indiscriminately return all zeros
>>>> + */
>>>>     
>>>>         
>>> Does it have to be so? It would be better to bounce reads to
>>> userspace...
>>>
>>>   
>>>       
>> Good idea.  I can set is_write = false and I should never get this
>> function called.
>>
>>     
>>>> +static void
>>>> +iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
>>>> +		      void *val)
>>>> +{
>>>> +	memset(val, 0, len);
>>>> +}
>>>> +
>>>> +/*
>>>> + * This function is called as KVM is completely shutting down.  We do not
>>>> + * need to worry about locking or careful RCU dancing...just nuke anything
>>>> + * we have as quickly as possible
>>>> + */
>>>> +static void
>>>> +iosignalfd_group_destructor(struct kvm_io_device *this)
>>>> +{
>>>> +	struct _iosignalfd_group *group = to_group(this);
>>>> +	struct _iosignalfd_item *item, *tmp;
>>>> +
>>>> +	list_for_each_entry_safe(item, tmp, &group->items, list) {
>>>> +		list_del(&item->list);
>>>> +		iosignalfd_item_free(item);
>>>> +	}
>>>> +
>>>> +	list_del(&group->list);
>>>> +	kfree(group);
>>>> +}
>>>> +
>>>> +static const struct kvm_io_device_ops iosignalfd_ops = {
>>>> +	.read       = iosignalfd_group_read,
>>>> +	.write      = iosignalfd_group_write,
>>>> +	.in_range   = iosignalfd_group_in_range,
>>>> +	.destructor = iosignalfd_group_destructor,
>>>> +};
>>>> +
>>>> +/* assumes kvm->lock held */
>>>> +static struct _iosignalfd_group *
>>>> +iosignalfd_group_find(struct kvm *kvm, u64 addr)
>>>> +{
>>>> +	struct _iosignalfd_group *group;
>>>> +
>>>> +	list_for_each_entry(group, &kvm->iosignalfds, list) {
>>>>     
>>>>         
>>> {} not needed here
>>>   
>>>       
>> Ack
>>     
>>>   
>>>       
>>>> +		if (group->addr == addr)
>>>> +			return group;
>>>> +	}
>>>> +
>>>> +	return NULL;
>>>> +}
>>>> +
>>>> +/* assumes kvm->lock is held */
>>>> +static struct _iosignalfd_group *
>>>> +iosignalfd_group_create(struct kvm *kvm, struct kvm_io_bus *bus,
>>>> +			u64 addr, size_t len)
>>>> +{
>>>> +	struct _iosignalfd_group *group;
>>>> +	int ret;
>>>> +
>>>> +	group = kzalloc(sizeof(*group), GFP_KERNEL);
>>>> +	if (!group)
>>>> +		return ERR_PTR(-ENOMEM);
>>>> +
>>>> +	INIT_LIST_HEAD(&group->list);
>>>> +	INIT_LIST_HEAD(&group->items);
>>>> +	group->addr   = addr;
>>>> +	group->length = len;
>>>> +	kvm_iodevice_init(&group->dev, &iosignalfd_ops);
>>>> +
>>>> +	ret = kvm_io_bus_register_dev(kvm, bus, &group->dev);
>>>> +	if (ret < 0) {
>>>> +		kfree(group);
>>>> +		return ERR_PTR(ret);
>>>> +	}
>>>> +
>>>> +	list_add_tail(&group->list, &kvm->iosignalfds);
>>>> +
>>>> +	return group;
>>>> +}
>>>> +
>>>> +static int
>>>> +kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
>>>> +{
>>>> +	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
>>>> +	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
>>>> +	struct _iosignalfd_group *group = NULL;
>>>>     
>>>>         
>>> why does group need to be initialized?
>>>
>>>   
>>>       
>>>> +	struct _iosignalfd_item  *item = NULL;
>>>>     
>>>>         
>>> Why does item need to be initialized?
>>>
>>>   
>>>       
>> Probably leftover from versions prior to v8.  Will fix.
>>
>>     
>>>> +	struct file              *file;
>>>> +	int                       ret;
>>>> +
>>>> +	if (args->len > sizeof(u64))
>>>>     
>>>>         
>>> Is e.g. value 3 legal?
>>>   
>>>       
>> Ack.  Will check against legal values.
>>
>>     
>>>   
>>>       
>>>> +		return -EINVAL;
>>>>     
>>>>         
>>>   
>>>       
>>>> +
>>>> +	file = eventfd_fget(args->fd);
>>>> +	if (IS_ERR(file))
>>>> +		return PTR_ERR(file);
>>>> +
>>>> +	item = kzalloc(sizeof(*item), GFP_KERNEL);
>>>> +	if (!item) {
>>>> +		ret = -ENOMEM;
>>>> +		goto fail;
>>>> +	}
>>>> +
>>>> +	INIT_LIST_HEAD(&item->list);
>>>> +	item->file = file;
>>>> +
>>>> +	/*
>>>> +	 * A trigger address is optional, otherwise this is a wildcard
>>>> +	 */
>>>> +	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER)
>>>> +		item->match = args->trigger;
>>>> +	else
>>>> +		item->wildcard = true;
>>>> +
>>>> +	mutex_lock(&kvm->lock);
>>>> +
>>>> +	/*
>>>> +	 * Put an upper limit on the number of items we support
>>>> +	 */
>>>>     
>>>>         
>>> Groups and items, actually, right?
>>>
>>>   
>>>       
>> Yeah, though technically that is implicit when you say "items", since
>> each group always has at least one item.  I will try to make this
>> clearer, though.
>>
>>     
>>>> +	if (kvm->io_device_count >= CONFIG_KVM_MAX_IO_DEVICES) {
>>>> +		ret = -ENOSPC;
>>>> +		goto unlock_fail;
>>>> +	}
>>>> +
>>>> +	group = iosignalfd_group_find(kvm, args->addr);
>>>> +	if (!group) {
>>>> +
>>>> +		group = iosignalfd_group_create(kvm, bus,
>>>> +						args->addr, args->len);
>>>> +		if (IS_ERR(group)) {
>>>> +			ret = PTR_ERR(group);
>>>> +			goto unlock_fail;
>>>> +		}
>>>> +
>>>> +		/*
>>>> +		 * Note: We do not increment io_device_count for the first item,
>>>> +		 * as this is represented by the group device that we just
>>>> +		 * registered.  Make sure we handle this properly when we
>>>> +		 * deassign the last item
>>>> +		 */
>>>> +	} else {
>>>> +
>>>> +		if (group->length != args->len) {
>>>> +			/*
>>>> +			 * Existing groups must have the same addr/len tuple
>>>> +			 * or we reject the request
>>>> +			 */
>>>> +			ret = -EINVAL;
>>>> +			goto unlock_fail;
>>>>     
>>>>         
>>> Most errors seem to trigger EINVAL. Applications will be
>>> easier to debug if different errors are returned on
>>> different mistakes.
>>>       
>> Yeah, agreed.  Will try to differentiate some errors here.
>>
>>     
>>>  E.g. here EBUSY might be good. And same
>>> in other places.
>>>
>>>   
>>>       
>> Actually, I think EBUSY is supposed to be a transitory error, and would
>> not be appropriate to use here.  That said, your point is taken: Find
>> more appropriate and descriptive errors.
>>
>>     
>>>> +		}
>>>> +
>>>> +		kvm->io_device_count++;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * Note: We are committed to succeed at this point since we have
>>>> +	 * (potentially) published a new group-device.  Any failure handling
>>>> +	 * added in the future after this point will need to be carefully
>>>> +	 * considered.
>>>> +	 */
>>>> +
>>>> +	list_add_tail_rcu(&item->list, &group->items);
>>>> +	group->count++;
>>>> +
>>>> +	mutex_unlock(&kvm->lock);
>>>> +
>>>> +	return 0;
>>>> +
>>>> +unlock_fail:
>>>> +	mutex_unlock(&kvm->lock);
>>>> +fail:
>>>> +	if (item)
>>>> +		/*
>>>> +		 * it would have never made it to the group->items list
>>>> +		 * in the failure path, so we dont need to worry about removing
>>>> +		 * it
>>>> +		 */
>>>> +		kfree(item);
>>>> +
>>>> +	fput(file);
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +
>>>> +static int
>>>> +kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
>>>> +{
>>>> +	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
>>>> +	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
>>>> +	struct _iosignalfd_group *group;
>>>> +	struct _iosignalfd_item  *item, *tmp;
>>>> +	struct file              *file;
>>>> +	int                       ret = 0;
>>>> +
>>>> +	file = eventfd_fget(args->fd);
>>>> +	if (IS_ERR(file))
>>>> +		return PTR_ERR(file);
>>>> +
>>>> +	mutex_lock(&kvm->lock);
>>>> +
>>>> +	group = iosignalfd_group_find(kvm, args->addr);
>>>> +	if (!group) {
>>>> +		ret = -EINVAL;
>>>> +		goto out;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * Exhaustively search our group->items list for any items that might
>>>> +	 * match the specified fd, and (carefully) remove each one found.
>>>> +	 */
>>>> +	list_for_each_entry_safe(item, tmp, &group->items, list) {
>>>> +
>>>> +		if (item->file != file)
>>>> +			continue;
>>>> +
>>>> +		list_del_rcu(&item->list);
>>>> +		group->count--;
>>>> +		if (group->count)
>>>> +			/*
>>>> +			 * We only decrement the global count if this is *not*
>>>> +			 * the last item.  The last item will be accounted for
>>>> +			 * by the io_bus_unregister
>>>> +			 */
>>>> +			kvm->io_device_count--;
>>>> +
>>>> +		/*
>>>> +		 * The item may be still referenced inside our group->write()
>>>> +		 * path's RCU read-side CS, so defer the actual free to the
>>>> +		 * next grace
>>>> +		 */
>>>> +		call_rcu(&item->rcu, iosignalfd_item_deferred_free);
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * Check if the group is now completely vacated as a result of
>>>> +	 * removing the items.  If so, unregister/delete it
>>>> +	 */
>>>> +	if (!group->count) {
>>>> +
>>>> +		kvm_io_bus_unregister_dev(kvm, bus, &group->dev);
>>>> +
>>>> +		/*
>>>> +		 * Like the item, the group may also still be referenced as
>>>> +		 * per above.  However, the kvm->iosignalfds list is not
>>>> +		 * RCU protected (its protected by kvm->lock instead) so
>>>> +		 * we can just plain-vanilla remove it.  What needs to be
>>>> +		 * done carefully is the actual freeing of the group pointer
>>>> +		 * since we walk the group->items list within the RCU CS.
>>>> +		 */
>>>> +		list_del(&group->list);
>>>> +		call_rcu(&group->rcu, iosignalfd_group_deferred_free);
>>>>     
>>>>         
>>> This is a deferred call, is it not, with no guarantee on when it will
>>> run? If correct I think synchronize_rcu might be better here:
>>> - can the module go away while iosignalfd_group_deferred_free is
>>>   running?
>>>   
>>>       
>> Good catch.  Once I go this route it will be easy to use SRCU instead of
>> RCU, too.  So I will fix this up.
>>
>>
>>     
>>> - can eventfd be signalled *after* ioctl exits? If yes
>>>   this might confuse applications if they use the eventfd
>>>   for something else.
>>>   
>>>       
>> Not by iosignalfd.  Once this function completes, we synchronously
>> guarantee that no more IO activity will generate an event on the
>> affected eventfds.  Of course, this has no bearing on whether some other
>> producer wont signal, but that is beyond the scope of iosignalfd.
>>     
>>>   
>>>       
>>>> +	}
>>>> +
>>>> +out:
>>>> +	mutex_unlock(&kvm->lock);
>>>> +
>>>> +	fput(file);
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +int
>>>> +kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
>>>> +{
>>>> +	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
>>>> +		return kvm_deassign_iosignalfd(kvm, args);
>>>> +
>>>> +	return kvm_assign_iosignalfd(kvm, args);
>>>> +}
>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>> index 42cbea7..e6495d4 100644
>>>> --- a/virt/kvm/kvm_main.c
>>>> +++ b/virt/kvm/kvm_main.c
>>>> @@ -971,7 +971,7 @@ static struct kvm *kvm_create_vm(void)
>>>>  	atomic_inc(&kvm->mm->mm_count);
>>>>  	spin_lock_init(&kvm->mmu_lock);
>>>>  	kvm_io_bus_init(&kvm->pio_bus);
>>>> -	kvm_irqfd_init(kvm);
>>>> +	kvm_eventfd_init(kvm);
>>>>  	mutex_init(&kvm->lock);
>>>>  	mutex_init(&kvm->irq_lock);
>>>>  	kvm_io_bus_init(&kvm->mmio_bus);
>>>> @@ -2227,6 +2227,15 @@ static long kvm_vm_ioctl(struct file *filp,
>>>>  		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
>>>>  		break;
>>>>  	}
>>>> +	case KVM_IOSIGNALFD: {
>>>> +		struct kvm_iosignalfd data;
>>>> +
>>>> +		r = -EFAULT;
>>>> +		if (copy_from_user(&data, argp, sizeof data))
>>>> +			goto out;
>>>> +		r = kvm_iosignalfd(kvm, &data);
>>>> +		break;
>>>> +	}
>>>>  #ifdef CONFIG_KVM_APIC_ARCHITECTURE
>>>>  	case KVM_SET_BOOT_CPU_ID:
>>>>  		r = 0;
>>>>
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>>>> the body of a message to majordomo@...r.kernel.org
>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>>     
>>>>         
>> Thanks Michael,
>> -Greg
>>
>>
>>     
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>   



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