Re: [PATCH RFC server v2 05/11] vfio-user: run vfio-user context

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Aug 27, 2021 at 01:53:24PM -0400, Jagannathan Raman wrote:
> Setup a handler to run vfio-user context. The context is driven by
> messages to the file descriptor associated with it - get the fd for
> the context and hook up the handler with it
> 
> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> ---
>  hw/remote/vfio-user-obj.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 70 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/remote/vfio-user-obj.c b/hw/remote/vfio-user-obj.c
> index 5ae0991..0726eb9 100644
> --- a/hw/remote/vfio-user-obj.c
> +++ b/hw/remote/vfio-user-obj.c
> @@ -35,6 +35,7 @@
>  #include "trace.h"
>  #include "sysemu/runstate.h"
>  #include "qemu/notify.h"
> +#include "qemu/thread.h"
>  #include "qapi/error.h"
>  #include "sysemu/sysemu.h"
>  #include "libvfio-user.h"
> @@ -65,6 +66,8 @@ struct VfuObject {
>      vfu_ctx_t *vfu_ctx;
>  
>      PCIDevice *pci_dev;
> +
> +    int vfu_poll_fd;
>  };
>  
>  static void vfu_object_set_socket(Object *obj, const char *str, Error **errp)
> @@ -89,13 +92,67 @@ static void vfu_object_set_devid(Object *obj, const char *str, Error **errp)
>      trace_vfu_prop("devid", str);
>  }
>  
> +static void vfu_object_ctx_run(void *opaque)
> +{
> +    VfuObject *o = opaque;
> +    int ret = -1;
> +
> +    while (ret != 0) {
> +        ret = vfu_run_ctx(o->vfu_ctx);
> +        if (ret < 0) {
> +            if (errno == EINTR) {
> +                continue;
> +            } else if (errno == ENOTCONN) {
> +                qemu_set_fd_handler(o->vfu_poll_fd, NULL, NULL, NULL);
> +                o->vfu_poll_fd = -1;
> +                object_unparent(OBJECT(o));
> +                break;
> +            } else {
> +                error_setg(&error_abort, "vfu: Failed to run device %s - %s",
> +                           o->devid, strerror(errno));
> +                 break;
> +            }
> +        }
> +    }
> +}
> +
> +static void *vfu_object_attach_ctx(void *opaque)
> +{
> +    VfuObject *o = opaque;
> +    int ret;
> +
> +retry_attach:
> +    ret = vfu_attach_ctx(o->vfu_ctx);
> +    if (ret < 0 && (errno == EAGAIN || errno == EWOULDBLOCK)) {

Does this loop consume 100% CPU since this is non-blocking?

Is it possible to register the fd with a QEMU AioContext instead of
spawning a separate thread?

libvfio-user has non-blocking listen_fd but conn_fd is always blocking.
This means ATTACH_NB is not useful because vfu_attach_ctx() is actually
blocking. I think this means vfu_run_ctx() is also blocking in some
places and QEMU's event loop might hang :(.

Can you make libvfio-user non-blocking in order to solve these issues?

> +        goto retry_attach;
> +    } else if (ret < 0) {
> +        error_setg(&error_abort,
> +                   "vfu: Failed to attach device %s to context - %s",
> +                   o->devid, strerror(errno));
> +        return NULL;
> +    }
> +
> +    o->vfu_poll_fd = vfu_get_poll_fd(o->vfu_ctx);
> +    if (o->vfu_poll_fd < 0) {
> +        error_setg(&error_abort, "vfu: Failed to get poll fd %s", o->devid);
> +        return NULL;
> +    }
> +
> +    qemu_set_fd_handler(o->vfu_poll_fd, vfu_object_ctx_run,
> +                        NULL, o);
> +
> +    return NULL;
> +}
> +
>  static void vfu_object_machine_done(Notifier *notifier, void *data)
>  {
>      VfuObject *o = container_of(notifier, VfuObject, machine_done);
>      DeviceState *dev = NULL;
> +    QemuThread thread;
>      int ret;
>  
> -    o->vfu_ctx = vfu_create_ctx(VFU_TRANS_SOCK, o->socket, 0,
> +    o->vfu_ctx = vfu_create_ctx(VFU_TRANS_SOCK, o->socket,
> +                                LIBVFIO_USER_FLAG_ATTACH_NB,
>                                  o, VFU_DEV_TYPE_PCI);
>      if (o->vfu_ctx == NULL) {
>          error_setg(&error_abort, "vfu: Failed to create context - %s",
> @@ -124,6 +181,16 @@ static void vfu_object_machine_done(Notifier *notifier, void *data)
>                     o->devid, strerror(errno));
>          return;
>      }
> +
> +    ret = vfu_realize_ctx(o->vfu_ctx);
> +    if (ret < 0) {
> +        error_setg(&error_abort, "vfu: Failed to realize device %s- %s",
> +                   o->devid, strerror(errno));
> +        return;
> +    }
> +
> +    qemu_thread_create(&thread, o->socket, vfu_object_attach_ctx, o,
> +                       QEMU_THREAD_DETACHED);

Is this thread leaked when the object is destroyed?

>  }
>  
>  static void vfu_object_init(Object *obj)
> @@ -147,6 +214,8 @@ static void vfu_object_init(Object *obj)
>  
>      o->machine_done.notify = vfu_object_machine_done;
>      qemu_add_machine_init_done_notifier(&o->machine_done);
> +
> +    o->vfu_poll_fd = -1;
>  }
>  
>  static void vfu_object_finalize(Object *obj)
> -- 
> 1.8.3.1
> 

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