Re: [Qemu-devel] [PATCH 0/2] virtio-pci: fix abort when fail to allocate ioeventfd

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Mar 14, 2012 at 9:22 AM, Avi Kivity <avi@redhat.com> wrote:
> On 03/13/2012 12:42 PM, Amos Kong wrote:
>> Boot up guest with 232 virtio-blk disk, qemu will abort for fail to
>> allocate ioeventfd. This patchset changes kvm_has_many_ioeventfds(),
>> and check if available ioeventfd exists. If not, virtio-pci will
>> fallback to userspace, and don't use ioeventfd for io notification.
>
> How about an alternative way of solving this, within the memory core:
> trap those writes in qemu and write to the ioeventfd yourself.  This way
> ioeventfds work even without kvm:
>
>
>  core: create eventfd
>  core: install handler for memory address that writes to ioeventfd
>  kvm (optional): install kernel handler for ioeventfd
>
> even if the third step fails, the ioeventfd still works, it's just slower.

That approach will penalize guests with large numbers of disks - they
see an extra switch to vcpu thread instead of kvm.ko -> iothread.  It
seems okay provided we can solve the limit in the kernel once and for
all by introducing a more dynamic data structure for in-kernel
devices.  That way future kernels will never hit an arbitrary limit
below their file descriptor rlimit.

Is there some reason why kvm.ko must use a fixed size array?  Would it
be possible to use a tree (maybe with a cache for recent lookups)?

Stefan