[Qemu-devel] Re: [PATCH] vhost: force vhost off for non-MSI guests

[Michael S. Tsirkin]

On Thu, Jan 20, 2011 at 08:31:53AM -0800, Sridhar Samudrala wrote:
> On Thu, 2011-01-20 at 17:35 +0200, Michael S. Tsirkin wrote:
> > When MSI is off, each interrupt needs to be bounced through the io
> > thread when it's set/cleared, so vhost-net causes more context switches and
> > higher CPU utilization than userspace virtio which handles networking in
> > the same thread.
> > 
> > We'll need to fix this by adding level irq support in kvm irqfd,
> > for now disable vhost-net in these configurations.
> > 
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> > 
> > I need to report some error from virtio-pci
> > that would be handled specially (disable but don't
> > report an error) so I wanted one that's never likely to be used by a
> > userspace ioctl. I selected ERANGE but it'd
> > be easy to switch to something else. Comments?
> 
> Should this error be EVHOST_DISABLED rather than EVIRTIO_DISABLED?
> 
> -Sridhar

The error is reported by virtio-pci which does not know about vhost.
I started with EVIRTIO_MSIX_DISABLED and made is shorter.
Would EVIRTIO_MSIX_DISABLED be better?

> > 
> >  hw/vhost.c      |    4 +++-
> >  hw/virtio-net.c |    6 ++++--
> >  hw/virtio-pci.c |    3 +++
> >  hw/virtio.h     |    2 ++
> >  4 files changed, 12 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/vhost.c b/hw/vhost.c
> > index 1d09ed0..c79765a 100644
> > --- a/hw/vhost.c
> > +++ b/hw/vhost.c
> > @@ -649,7 +649,9 @@ int vhost_dev_start(struct vhost_dev *hdev, 
> > VirtIODevice *vdev)
> >  
> >      r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
> >      if (r < 0) {
> > -        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
> > +   if (r != -EVIRTIO_DISABLED) {
> > +           fprintf(stderr, "Error binding guest notifier: %d\n", -r);
> > +   }
> >          goto fail_notifiers;
> >      }
> >  
> > diff --git a/hw/virtio-net.c b/hw/virtio-net.c
> > index ccb3e63..5de3fee 100644
> > --- a/hw/virtio-net.c
> > +++ b/hw/virtio-net.c
> > @@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, 
> > uint8_t status)
> >      if (!n->vhost_started) {
> >          int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), 
> > &n->vdev);
> >          if (r < 0) {
> > -            error_report("unable to start vhost net: %d: "
> > -                         "falling back on userspace virtio", -r);
> > +            if (r != -EVIRTIO_DISABLED) {
> > +                error_report("unable to start vhost net: %d: "
> > +                             "falling back on userspace virtio", -r);
> > +            }
> >          } else {
> >              n->vhost_started = 1;
> >          }
> > diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
> > index dd8887a..dbf4be0 100644
> > --- a/hw/virtio-pci.c
> > +++ b/hw/virtio-pci.c
> > @@ -628,6 +628,9 @@ static int virtio_pci_set_guest_notifier(void *opaque, 
> > int n, bool assign)
> >      EventNotifier *notifier = virtio_queue_get_guest_notifier(vq);
> >  
> >      if (assign) {
> > +        if (!msix_enabled(&proxy->pci_dev)) {
> > +            return -EVIRTIO_DISABLED;
> > +        }
> >          int r = event_notifier_init(notifier, 0);
> >          if (r < 0) {
> >              return r;
> > diff --git a/hw/virtio.h b/hw/virtio.h
> > index d8546d5..53bbdba 100644
> > --- a/hw/virtio.h
> > +++ b/hw/virtio.h
> > @@ -98,6 +98,8 @@ typedef struct {
> >      void (*vmstate_change)(void * opaque, bool running);
> >  } VirtIOBindings;
> >  
> > +#define EVIRTIO_DISABLED ERANGE
> > +
> >  #define VIRTIO_PCI_QUEUE_MAX 64
> >  
> >  #define VIRTIO_NO_VECTOR 0xffff