On Wed, Mar 14, 2012 at 10:05 AM, Avi Kivity <avi@xxxxxxxxxx> wrote: > On 03/14/2012 11:59 AM, Stefan Hajnoczi wrote: >> On Wed, Mar 14, 2012 at 9:22 AM, Avi Kivity <avi@xxxxxxxxxx> 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's the normal path when you attach >232 virtio-blk devices to a guest (or 300 in the future). >> 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. Shouldn't the limit be the file descriptor rlimit? If userspace cannot create more eventfds then it cannot set up more ioeventfds. I agree there always needs to be an error path because there is a finite resource (either file descriptors or in-kernel device slots). Stefan -- 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
