Michael S. Tsirkin wrote:
> On Thu, Jun 25, 2009 at 09:28:32AM -0400, Gregory Haskins wrote:
>
>> DEASSIGN allows us to optionally disassociate an IRQFD from its underlying
>> eventfd without destroying the eventfd in the process. This is useful
>> for conditions like live-migration which may have an eventfd associated
>> with a device and an IRQFD. We need to be able to decouple the guest
>> from the event source while not perturbing the event source itself.
>>
>> Signed-off-by: Gregory Haskins <ghask...@novell.com>
>> CC: Michael S. Tsirkin <m...@redhat.com>
>> ---
>>
>> include/linux/kvm.h | 2 ++
>> virt/kvm/eventfd.c | 56
>> +++++++++++++++++++++++++++++++++++++++++++++++++--
>> 2 files changed, 56 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
>> index 38ff31e..6710518 100644
>> --- a/include/linux/kvm.h
>> +++ b/include/linux/kvm.h
>> @@ -490,6 +490,8 @@ struct kvm_x86_mce {
>> };
>> #endif
>>
>> +#define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
>> +
>> struct kvm_irqfd {
>> __u32 fd;
>> __u32 gsi;
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index ca21e8a..2d4549c 100644
>> --- a/virt/kvm/eventfd.c
>> +++ b/virt/kvm/eventfd.c
>> @@ -180,8 +180,8 @@ irqfd_ptable_queue_proc(struct file *file,
>> wait_queue_head_t *wqh,
>> add_wait_queue(wqh, &irqfd->wait);
>> }
>>
>> -int
>> -kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
>> +static int
>> +kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>> {
>> struct _irqfd *irqfd;
>> struct file *file = NULL;
>> @@ -303,6 +303,58 @@ irqfd_shutdown(struct _irqfd *irqfd)
>> irqfd_release(irqfd);
>> }
>>
>> +/*
>> + * assumes kvm->irqfds.lock is held
>> + */
>> +static struct _irqfd *
>> +irqfd_find(struct kvm *kvm, int fd, int gsi)
>> +{
>> + struct _irqfd *irqfd, *tmp, *ret = ERR_PTR(-ENOENT);
>> + struct eventfd_ctx *eventfd;
>> +
>> + eventfd = eventfd_ctx_fdget(fd);
>> + if (IS_ERR(eventfd))
>> + return ERR_PTR(PTR_ERR(eventfd));
>> +
>> + spin_lock_irq(&kvm->irqfds.lock);
>> +
>> + list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds.items, list) {
>> + if (irqfd->eventfd == eventfd && irqfd->gsi == gsi) {
>> + irqfd_deactivate(irqfd);
>> + ret = irqfd;
>> + break;
>> + }
>> + }
>> +
>> + spin_unlock_irq(&kvm->irqfds.lock);
>> + eventfd_ctx_put(eventfd);
>> +
>> + return ret;
>> +}
>> +
>> +static int
>> +kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
>> +{
>> + struct _irqfd *irqfd;
>> +
>> + irqfd = irqfd_find(kvm, fd, gsi);
>> + if (IS_ERR(irqfd))
>> + return PTR_ERR(irqfd);
>> +
>> + irqfd_shutdown(irqfd);
>> +
>> + return 0;
>> +}
>>
>
>
> I think that, to make this work properly, you must
> add irqfd to list the last thing so do.
> As it is, when you assign irqfd, the last thing you do is
>
> irqfd->eventfd = eventfd;
>
Yeah, I agree. I actually already replied to this effect on the thread
for 3/4. ;)
> I think you should move this to within a spinlock.
>
I think if I fix the ordering, the list spinlock should be sufficient.
Am I missing something?
>
>> +
>> +int
>> +kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
>> +{
>> + if (flags & KVM_IRQFD_FLAG_DEASSIGN)
>> + return kvm_irqfd_deassign(kvm, fd, gsi);
>> +
>> + return kvm_irqfd_assign(kvm, fd, gsi);
>> +}
>> +
>>
>
>
> At some point we discussed limiting the number of
> irqfds that can be created in some way, so that userspace
> can not consume unlimited amount of kernel memory.
>
> What happened to that idea?
>
Yeah, that is a good question. I thought I had already done that, too,
but now I don't know what happened to the logic. Perhaps it got lost on
a respin somewhere. I will look into this and add the feature.
> This will happen naturally if
> - we keep fget on the file until irqfd goes away
> - we allow the same file be bound to only one irqfd
> but there might be other ways to do this
>
>
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