[Qemu-devel] Re: [PATCHv2] vhost: force vhost off for non-MSI guests

[Michael S. Tsirkin]

On Mon, Jan 31, 2011 at 02:47:34PM -0700, Alex Williamson wrote:
> On Mon, 2011-01-31 at 23:19 +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.
> > 
> > Added a vhostforce flag to force vhost-net back on.
> > 
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> > 
> > Untested, I'll send a pull request later after
> > some testing assuming we've resolved the
> > command line comments to everyone's satisfaction.
> > 
> > Changes since v1:
> >     added vhostforce to enable vhost for non-MSI guests
> 
> I'm still not thrilled with the errno hack.  If this is really a generic
> problem, we should better architect the set_guest_notifiers() callback,
> the force option feels like a band-aide.
>  I'm still curious if we can't
> be more dynamic about how we setup the set_guest_notifiers() function
> pointer so it only gets set if (force || msix_enabled()) and we can
> limit some of the impact here to vhost.  Thanks,
> 
> Alex

I'm not entirely happy with the API either, but I'm sure
playing with the function pointer is not the solution
we are looking for:

msix is enabled/disabled dynamically.  force is static but it is
something net backend knows about, virtio pci doesn't.
So I suspect modifying the callback on the fly will become
messy quite quickly.

Well, this API will be easy to tweak after the fact, there's a single
user after all.

> >  hw/vhost.c      |   16 +++++++++++-----
> >  hw/vhost.h      |    3 ++-
> >  hw/vhost_net.c  |    8 +++++---
> >  hw/vhost_net.h  |    2 +-
> >  hw/virtio-net.c |    6 ++++--
> >  hw/virtio-pci.c |    6 +++++-
> >  hw/virtio.h     |    4 +++-
> >  net/tap.c       |    6 ++++--
> >  qemu-options.hx |    4 +++-
> >  9 files changed, 38 insertions(+), 17 deletions(-)
> > 
> > diff --git a/hw/vhost.c b/hw/vhost.c
> > index 6082da2..72df75f 100644
> > --- a/hw/vhost.c
> > +++ b/hw/vhost.c
> > @@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev 
> > *dev,
> >                                0, virtio_queue_get_desc_size(vdev, idx));
> >  }
> >  
> > -int vhost_dev_init(struct vhost_dev *hdev, int devfd)
> > +int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
> >  {
> >      uint64_t features;
> >      int r;
> > @@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
> >      hdev->log_enabled = false;
> >      hdev->started = false;
> >      cpu_register_phys_memory_client(&hdev->client);
> > +    hdev->force = force;
> >      return 0;
> >  fail:
> >      r = -errno;
> > @@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, 
> > VirtIODevice *vdev)
> >          goto fail;
> >      }
> >  
> > -    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
> > +    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
> > +                                           hdev->force);
> >      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;
> >      }
> >  
> > @@ -686,7 +690,8 @@ fail_vq:
> >      }
> >  fail_mem:
> >  fail_features:
> > -    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
> > +    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
> > +                                       hdev->force);
> >  fail_notifiers:
> >  fail:
> >      return r;
> > @@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, 
> > VirtIODevice *vdev)
> >      }
> >      vhost_client_sync_dirty_bitmap(&hdev->client, 0,
> >                                     (target_phys_addr_t)~0x0ull);
> > -    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
> > +    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
> > +                                           hdev->force);
> >      if (r < 0) {
> >          fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
> >          fflush(stderr);
> > diff --git a/hw/vhost.h b/hw/vhost.h
> > index 86dd834..d5d4d86 100644
> > --- a/hw/vhost.h
> > +++ b/hw/vhost.h
> > @@ -38,9 +38,10 @@ struct vhost_dev {
> >      bool log_enabled;
> >      vhost_log_chunk_t *log;
> >      unsigned long long log_size;
> > +    bool force;
> >  };
> >  
> > -int vhost_dev_init(struct vhost_dev *hdev, int devfd);
> > +int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
> >  void vhost_dev_cleanup(struct vhost_dev *hdev);
> >  int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
> >  void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);
> > diff --git a/hw/vhost_net.c b/hw/vhost_net.c
> > index c068be1..4f57271 100644
> > --- a/hw/vhost_net.c
> > +++ b/hw/vhost_net.c
> > @@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
> >      }
> >  }
> >  
> > -struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
> > +struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
> > +                                 bool force)
> >  {
> >      int r;
> >      struct vhost_net *net = qemu_malloc(sizeof *net);
> > @@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState 
> > *backend, int devfd)
> >          (1 << VHOST_NET_F_VIRTIO_NET_HDR);
> >      net->backend = r;
> >  
> > -    r = vhost_dev_init(&net->dev, devfd);
> > +    r = vhost_dev_init(&net->dev, devfd, force);
> >      if (r < 0) {
> >          goto fail;
> >      }
> > @@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
> >      qemu_free(net);
> >  }
> >  #else
> > -struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
> > +struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
> > +                                 bool force)
> >  {
> >     return NULL;
> >  }
> > diff --git a/hw/vhost_net.h b/hw/vhost_net.h
> > index 6c18ff7..8435094 100644
> > --- a/hw/vhost_net.h
> > +++ b/hw/vhost_net.h
> > @@ -6,7 +6,7 @@
> >  struct vhost_net;
> >  typedef struct vhost_net VHostNetState;
> >  
> > -VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
> > +VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool 
> > force);
> >  
> >  int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
> >  void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);
> > 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 d07ff97..c93ef11 100644
> > --- a/hw/virtio-pci.c
> > +++ b/hw/virtio-pci.c
> > @@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void 
> > *opaque, int n, bool assign)
> >      return 0;
> >  }
> >  
> > -static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
> > +static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool 
> > force)
> >  {
> >      VirtIOPCIProxy *proxy = opaque;
> >      VirtIODevice *vdev = proxy->vdev;
> >      int r, n;
> >  
> > +    if (assign && !force && !msix_enabled(&proxy->pci_dev)) {
> > +       return -EVIRTIO_DISABLED;
> > +    }
> > +
> >      for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
> >          if (!virtio_queue_get_num(vdev, n)) {
> >              break;
> > diff --git a/hw/virtio.h b/hw/virtio.h
> > index d8546d5..156c9a3 100644
> > --- a/hw/virtio.h
> > +++ b/hw/virtio.h
> > @@ -93,11 +93,13 @@ typedef struct {
> >      int (*load_config)(void * opaque, QEMUFile *f);
> >      int (*load_queue)(void * opaque, int n, QEMUFile *f);
> >      unsigned (*get_features)(void * opaque);
> > -    int (*set_guest_notifiers)(void * opaque, bool assigned);
> > +    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
> >      int (*set_host_notifier)(void * opaque, int n, bool assigned);
> >      void (*vmstate_change)(void * opaque, bool running);
> >  } VirtIOBindings;
> >  
> > +#define EVIRTIO_DISABLED ERANGE
> > +
> >  #define VIRTIO_PCI_QUEUE_MAX 64
> >  
> >  #define VIRTIO_NO_VECTOR 0xffff
> > diff --git a/net/tap.c b/net/tap.c
> > index eada34a..b8cd252 100644
> > --- a/net/tap.c
> > +++ b/net/tap.c
> > @@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const 
> > char *name, VLANState *vlan
> >          }
> >      }
> >  
> > -    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) 
> > {
> > +    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
> > +                          qemu_opt_get_bool(opts, "vhostforce", false))) {
> >          int vhostfd, r;
> > +        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
> >          if (qemu_opt_get(opts, "vhostfd")) {
> >              r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
> >              if (r == -1) {
> > @@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const 
> > char *name, VLANState *vlan
> >          } else {
> >              vhostfd = -1;
> >          }
> > -        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
> > +        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
> >          if (!s->vhost_net) {
> >              error_report("vhost-net requested but could not be 
> > initialized");
> >              return -1;
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 898561d..11c93a2 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
> >      "-net tap[,vlan=n][,name=str],ifname=name\n"
> >      "                connect the host TAP network interface to VLAN 'n'\n"
> >  #else
> > -    "-net 
> > tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
> > +    "-net 
> > tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
> >      "                connect the host TAP network interface to VLAN 'n' 
> > and use the\n"
> >      "                network scripts 'file' (default=" 
> > DEFAULT_NETWORK_SCRIPT ")\n"
> >      "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT 
> > ")\n"
> > @@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
> >      "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR 
> > tap flag\n"
> >      "                use vnet_hdr=on to make the lack of IFF_VNET_HDR 
> > support an error condition\n"
> >      "                use vhost=on to enable experimental in kernel 
> > accelerator\n"
> > +    "                    (only has effect for virtio guests which use 
> > MSIX)\n"
> > +    "                use vhostforce=on to force vhost on for non-MSIX 
> > virtio guests\n"
> >      "                use 'vhostfd=h' to connect to an already opened vhost 
> > net device\n"
> >  #endif
> >      "-net 
> > socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"
> 
>