device limit for kvm_io_bus

device limit for kvm_io_bus

[Cam Macdonell]

Hi,

I'm trying to use ioeventfds for notification between guests.  After
assigning a handful of ioeventfds I was getting a "no space left on
device error".  The culprit seems to be that only 6 devices are
allowed for a guest on the kvm IO bus.  The comment indicates a
somewhat low number was chosen.  Can the limit be increased to
something on the order of the number of file descriptors a process
could have?

/*
 * It would be nice to use something smarter than a linear search, TBD...
 * Thankfully we dont expect many devices to register (famous last words :),
 * so until then it will suffice.  At least its abstracted so we can change
 * in one place.
 */
struct kvm_io_bus {
	int                   dev_count;
#define NR_IOBUS_DEVS 6
	struct kvm_io_device *devs[NR_IOBUS_DEVS];
};

from kvm_main.c
int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
			    struct kvm_io_device *dev)
{
	struct kvm_io_bus *new_bus, *bus;

	bus = kvm->buses[bus_idx];
	if (bus->dev_count > NR_IOBUS_DEVS-1)
		return -ENOSPC;

...snip...

}

Thanks,
Cam

Re: device limit for kvm_io_bus

[Avi Kivity]

On 03/30/2010 06:56 PM, Cam Macdonell wrote:
> Hi,
>
> I'm trying to use ioeventfds for notification between guests.  After
> assigning a handful of ioeventfds I was getting a "no space left on
> device error".  The culprit seems to be that only 6 devices are
> allowed for a guest on the kvm IO bus.  The comment indicates a
> somewhat low number was chosen.  Can the limit be increased to
> something on the order of the number of file descriptors a process
> could have?
>    

There's in fact a patch on the list to do just that, at least three 
people have been hitting it simultaneously.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.