Re: [Qemu-devel] [PATCH] virtio-9p: add reset handler

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 06, 2016 at 03:12:10PM +0200, Greg Kurz wrote:
> Virtio devices should implement the VirtIODevice->reset() function to
> perform necessary cleanup actions and to bring the device to a quiescent
> state.
> 
> In the case of the virtio-9p device, this means:
> - emptying the list of active PDUs (i.e. draining all in-flight I/O)
> - freeing all fids (i.e. close open file descriptors and free memory)
> 
> That's what this patch does.
> 
> The reset handler first waits for all active PDUs to complete. Since
> completion happens in the QEMU global aio context, we just have to
> loop around aio_poll() until the active list is empty.
> 
> The freeing part involves some actions to be performed on the backend,
> like closing file descriptors or flushing extended attributes to the
> underlying filesystem. The virtfs_reset() function already does the
> job: it calls free_fid() for all open fids not involved in an ongoing
> I/O operation. We are sure this is the case since we have drained
> the PDU active list.
> 
> The current code implements all backend accesses with coroutines, but we
> want to stay synchronous on the reset path. We can either change the
> current code to be able to run when not in coroutine context, or create
> a coroutine context and wait for virtfs_reset() to complete. This patch
> goes for the latter because it results in simpler code.
> 
> Note that we also need to create a dummy PDU because it is also an API
> to pass the FsContext pointer to all backend callbacks.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  hw/9pfs/9p.c               |   31 +++++++++++++++++++++++++++++++
>  hw/9pfs/9p.h               |    1 +
>  hw/9pfs/virtio-9p-device.c |    8 ++++++++
>  3 files changed, 40 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 119ee584969b..42137395037e 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -3522,6 +3522,37 @@ void v9fs_device_unrealize_common(V9fsState *s, Error **errp)
>      g_free(s->tag);
>  }
>  
> +
> +typedef struct VirtfsCoResetData {
> +    V9fsPDU pdu;
> +    bool done;
> +} VirtfsCoResetData;
> +
> +static void coroutine_fn virtfs_co_reset(void *opaque)
> +{
> +    VirtfsCoResetData *data = opaque;
> +
> +    virtfs_reset(&data->pdu);
> +    data->done = true;
> +}
> +
> +void v9fs_reset(V9fsState *s)
> +{
> +    VirtfsCoResetData data = { .pdu = { .s = s }, .done = false };
> +    Coroutine *co;
> +
> +    while (!QLIST_EMPTY(&s->active_list)) {
> +        aio_poll(qemu_get_aio_context(), true);
> +    }
> +
> +    co = qemu_coroutine_create(virtfs_co_reset, &data);
> +    qemu_coroutine_enter(co);
> +
> +    while (!data.done) {
> +        aio_poll(qemu_get_aio_context(), true);
> +    }
> +}
> +
>  static void __attribute__((__constructor__)) v9fs_set_fd_limit(void)
>  {
>      struct rlimit rlim;
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index d539d2ebe9c0..6b69eaf24614 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -339,5 +339,6 @@ ssize_t pdu_unmarshal(V9fsPDU *pdu, size_t offset, const char *fmt, ...);
>  V9fsPDU *pdu_alloc(V9fsState *s);
>  void pdu_free(V9fsPDU *pdu);
>  void pdu_submit(V9fsPDU *pdu);
> +void v9fs_reset(V9fsState *s);
>  
>  #endif
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index 009b43f6d045..b73d72aceb64 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -130,6 +130,13 @@ static void virtio_9p_device_unrealize(DeviceState *dev, Error **errp)
>      v9fs_device_unrealize_common(s, errp);
>  }
>  
> +static void virtio_9p_reset(VirtIODevice *vdev)
> +{
> +    V9fsVirtioState *v = (V9fsVirtioState *)vdev;
> +
> +    v9fs_reset(&v->state);
> +}
> +
>  ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
>                              const char *fmt, va_list ap)
>  {
> @@ -188,6 +195,7 @@ static void virtio_9p_class_init(ObjectClass *klass, void *data)
>      vdc->unrealize = virtio_9p_device_unrealize;
>      vdc->get_features = virtio_9p_get_features;
>      vdc->get_config = virtio_9p_get_config;
> +    vdc->reset = virtio_9p_reset;
>  }
>  
>  static const TypeInfo virtio_device_info = {