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

[Alex Williamson]

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

>  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"