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

From: Michael S. Tsirkin
Date: Mon Jun 22 2009 - 09:09:19 EST

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On Mon, Jun 22, 2009 at 08:56:28AM -0400, Gregory Haskins wrote:
> 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.

Ah, I see now. Naturally it is really up to you.

> I can fix these if they bother you, but I suspect just
> about every comment I've written has them too. ;)
>
> -Greg

It doesn't bother me as such. But you seem to care about such things :).
If you do care, other comments in kvm don't seem to be like this and
people won't remember to add spaces in comments, though.

>
> >
> >>>> + */
> >>>> +
> >>>> +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@xxxxxxxxxxxxxxx
> >>>> 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@xxxxxxxxxxxxxxx
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
> >
>
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