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

[Amos Kong]

----- Original Message -----
> On Tue, Mar 13, 2012 at 2:47 PM, Amos Kong <address@hidden> wrote:

...

Hi, Stefan

> > diff --git a/kvm-all.c b/kvm-all.c
> > index 77eadf6..7157e78 100644
> > --- a/kvm-all.c
> > +++ b/kvm-all.c
> > @@ -771,6 +771,8 @@ static void
> > kvm_io_ioeventfd_add(MemoryRegionSection *sectio
> >
> >     r = kvm_set_ioeventfd_pio_word(fd,
> >     section->offset_within_address_space,
> >                                    data, true);
> > +    if (r == -ENOSPC)
> > +        return;
>
> The caller needs a way to detect the failure.

Yes, about memory API

> >     if (r < 0) {
> >         abort();
> >     }
> >
> >
> >
> >> Basically we need a way for ioeventfd to fail if we hit rlimit or
> >> the
> >> in-kernel io bus device limit.  Suggestions?
> >
> >
> >
> >> (The reason I don't like using has_many_ioeventfds() is because
> >> it's
> >> ugly and inefficient to open and then close file descriptors -
> >> especially in a multithreaded program where file descriptor
> >> availability can change while we're executing.)
> >
> > I identified it's too ugly ;)
> > but I want to reserve it for older kernel (iobus limit is 6)
> >
> > Can we remove the check for old kernel (iobus limit is 6)?
> > user can disable ioeventfd through qemu cmdline
> >  virtio-net-pci.ioeventfd=on/off
> >  virtio-blk-pci.ioeventfd=on/off
>
> Why do you want to remove the iobus limit 6 check?  The point of that
> check is to allow vhost-net to always work since it requires an
> ioeventfd.


### -device virtio-blk-pci,ioeventfd=off,drive=drive-virtio0-0-0,id=id1 -drive 
...
     this blk dev will not use ioeventfd for io notification.

### -device virtio-net-pci,netdev=he,ioeventfd=off -netdev tap,id=he 
     this net dev will not use ioeventfd

### -device virtio-net-pci,netdev=he,vhost=on -netdev tap,id=he
     this dev will take 2 ioeventfd (service notifications from rx/tx queues)

ioeventfd paramenter is a way for user to limit ioeventfd usage.


### qemu-kvm -net none -device 
virtio-blk-pci,ioeventfd=on,drive=drive-virtio0-0-0,id=id1 -drive ...
For some odd users, they don't use network, and they need a better disk io 
performance.
but we could not satisfy them if we reserve the checking of 6 iobus devs.

Strategy should be decided by users.

> Userspace virtio devices, on the other hand, can
> gracefully fall back to non-ioeventfd.

This is expected, not abort.


Thanks, Amos.