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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Mar 16, 2012 at 04:59:35PM +0800, Amos Kong wrote:
> On 14/03/12 19:46, Stefan Hajnoczi wrote:
> >On Wed, Mar 14, 2012 at 10:46 AM, Avi Kivity<avi@redhat.com>  wrote:
> >>On 03/14/2012 12:39 PM, Stefan Hajnoczi wrote:
> >>>On Wed, Mar 14, 2012 at 10:05 AM, Avi Kivity<avi@redhat.com>  wrote:
> >>>>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
> 
> Can you give some detail about this? I'm not familiar with Memory API.
> 
> 
> btw, can we fix this problem by replacing abort() by a error note?
> virtio-pci will auto fallback to userspace.
> 
> diff --git a/kvm-all.c b/kvm-all.c
> index 3c6b4f0..cf23dbf 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -749,7 +749,8 @@ static void
> kvm_mem_ioeventfd_add(MemoryRegionSection *section,
>      r = kvm_set_ioeventfd_mmio_long(fd,
> section->offset_within_address_space,
>                                      data, true);
>      if (r < 0) {
> -        abort();
> +        fprintf(stderr, "%s: unable to map ioeventfd: %s.\nFallback to "
> +                "userspace (slower).\n", __func__, strerror(-r));

The challenge is propagating the error code.  If virtio-pci.c doesn't
know that ioeventfd has failed, then it's not possible to fall back to a
userspace handler.

I believe Avi's suggestion is to put the fallback code into the KVM
memory API implementation so that virtio-pci.c doesn't need to know that
ioeventfd failed at all.

Stefan