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

[Avi Kivity <avi@redhat.com>]

On 03/14/2012 11:59 AM, Stefan Hajnoczi wrote:
> 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's only a failure path.  The normal path is expected to have a kvm
ioeventfd installed.

>   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)?

It does use bsearch today IIRC.  We'll expand the limit, but there must
be a limit, and qemu must be prepared to deal with it.

-- 
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