Re: [KVM PATCH v8 3/3] KVM: add iosignalfd support

Gregory Haskins
2009-06-22T12:56:31Z

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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@vger.kernel.org
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>>>>
>>>>
>> Thanks Michael,
>> -Greg
>>
>>
>>
>
>
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