Re: [Qemu-devel] [PATCH V2] vhost: logs sharing

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

On Fri, Apr 10, 2015 at 05:33:35PM +0800, Jason Wang wrote:
> Currently we allocate one vhost log per vhost device. This is sub
> optimal when:
> 
> - Guest has several device with vhost as backend
> - Guest has multiqueue devices
> 
> In the above cases, we can avoid the memory allocation by sharing a
> single vhost log among all the vhost devices. This is done through:
> 
> - Introducing a new vhost_log structure with refcnt inside.
> - Using a global pointer to vhost_log structure that will be used. And
>   introduce helper to get the log with expected log size and helper to
> - drop the refcnt to the old log.
> - Each vhost device still keep track of a pointer to the log that was
>   used.
> 
> With above, if no resize happens, all vhost device will share a single
> vhost log. During resize, a new vhost_log structure will be allocated
> and made for the global pointer. And each vhost devices will drop the
> refcnt to the old log.
> 
> Tested by doing scp during migration for a 2 queues virtio-net-pci.
> 
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> Changes from V1:
> - Drop the list of vhost log, instead, using a global pointer instead

I don't think it works like this. If you have a global pointer,
you also need a global listener, have that sync all devices.


> ---
>  hw/virtio/vhost.c         | 66 ++++++++++++++++++++++++++++++++++++++---------
>  include/hw/virtio/vhost.h |  9 ++++++-
>  2 files changed, 62 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 5a12861..e16c2db 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -22,15 +22,19 @@
>  #include "hw/virtio/virtio-bus.h"
>  #include "migration/migration.h"
>  
> +static struct vhost_log *vhost_log;
> +
>  static void vhost_dev_sync_region(struct vhost_dev *dev,
>                                    MemoryRegionSection *section,
>                                    uint64_t mfirst, uint64_t mlast,
>                                    uint64_t rfirst, uint64_t rlast)
>  {
> +    vhost_log_chunk_t *log = dev->log->log;
> +
>      uint64_t start = MAX(mfirst, rfirst);
>      uint64_t end = MIN(mlast, rlast);
> -    vhost_log_chunk_t *from = dev->log + start / VHOST_LOG_CHUNK;
> -    vhost_log_chunk_t *to = dev->log + end / VHOST_LOG_CHUNK + 1;
> +    vhost_log_chunk_t *from = log + start / VHOST_LOG_CHUNK;
> +    vhost_log_chunk_t *to = log + end / VHOST_LOG_CHUNK + 1;
>      uint64_t addr = (start / VHOST_LOG_CHUNK) * VHOST_LOG_CHUNK;
>  
>      if (end < start) {
> @@ -280,22 +284,55 @@ static uint64_t vhost_get_log_size(struct vhost_dev *dev)
>      }
>      return log_size;
>  }
> +static struct vhost_log *vhost_log_alloc(uint64_t size)
> +{
> +    struct vhost_log *log = g_malloc0(sizeof *log + size * sizeof(*(log->log)));
> +
> +    log->size = size;
> +    log->refcnt = 1;
> +
> +    return log;
> +}
> +
> +static struct vhost_log *vhost_log_get(uint64_t size)
> +{
> +    if (!vhost_log || vhost_log->size != size) {
> +        vhost_log = vhost_log_alloc(size);

This just leaks the old log if size != size.

> +    } else {
> +        ++vhost_log->refcnt;
> +    }
> +
> +    return vhost_log;
> +}
> +
> +static void vhost_log_put(struct vhost_log *log)
> +{
> +    if (!log) {
> +        return;
> +    }
> +
> +    --log->refcnt;
> +    if (log->refcnt == 0) {
> +        if (vhost_log == log) {
> +            vhost_log = NULL;
> +        }
> +        g_free(log);
> +    }
> +}
>  
>  static inline void vhost_dev_log_resize(struct vhost_dev* dev, uint64_t size)
>  {
> -    vhost_log_chunk_t *log;
> -    uint64_t log_base;
> +    struct vhost_log *log = vhost_log_get(size);

At this point next device will try to use the
new log, but there is nothing there.

> +    uint64_t log_base = (uint64_t)(unsigned long)log->log;
>      int r;
>  
> -    log = g_malloc0(size * sizeof *log);
> -    log_base = (uint64_t)(unsigned long)log;
>      r = dev->vhost_ops->vhost_call(dev, VHOST_SET_LOG_BASE, &log_base);
>      assert(r >= 0);
>      /* Sync only the range covered by the old log */
>      if (dev->log_size) {
>          vhost_log_sync_range(dev, 0, dev->log_size * VHOST_LOG_CHUNK - 1);
>      }
> -    g_free(dev->log);
> +    vhost_log_put(dev->log);
>      dev->log = log;
>      dev->log_size = size;
>  }
> @@ -601,7 +638,7 @@ static int vhost_migration_log(MemoryListener *listener, int enable)
>          if (r < 0) {
>              return r;
>          }
> -        g_free(dev->log);
> +        vhost_log_put(dev->log);
>          dev->log = NULL;
>          dev->log_size = 0;
>      } else {
> @@ -1058,9 +1095,11 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>  
>      if (hdev->log_enabled) {
>          hdev->log_size = vhost_get_log_size(hdev);
> -        hdev->log = hdev->log_size ?
> -            g_malloc0(hdev->log_size * sizeof *hdev->log) : NULL;
> -        r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE, hdev->log);
> +        if (hdev->log_size) {
> +            hdev->log = vhost_log_get(hdev->log_size);
> +        }
> +        r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE,
> +                                        hdev->log_size ? hdev->log->log : NULL);
>          if (r < 0) {
>              r = -errno;
>              goto fail_log;
> @@ -1069,6 +1108,9 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>  
>      return 0;
>  fail_log:
> +    if (hdev->log_size) {
> +        vhost_log_put(hdev->log);
> +    }
>  fail_vq:
>      while (--i >= 0) {
>          vhost_virtqueue_stop(hdev,
> @@ -1098,7 +1140,7 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
>      vhost_log_sync_range(hdev, 0, ~0x0ull);
>  
>      hdev->started = false;
> -    g_free(hdev->log);
> +    vhost_log_put(hdev->log);
>      hdev->log = NULL;
>      hdev->log_size = 0;
>  }
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index d5593d1..3879778 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -28,6 +28,13 @@ typedef unsigned long vhost_log_chunk_t;
>  #define VHOST_LOG_CHUNK (VHOST_LOG_PAGE * VHOST_LOG_BITS)
>  #define VHOST_INVALID_FEATURE_BIT   (0xff)
>  
> +struct vhost_log {
> +    QLIST_ENTRY(vhost_log) node;
> +    unsigned long long size;
> +    int refcnt;
> +    vhost_log_chunk_t log[0];
> +};
> +

Pls fix coding style here: typedef ... VhostLog.

>  struct vhost_memory;
>  struct vhost_dev {
>      MemoryListener memory_listener;
> @@ -43,7 +50,6 @@ struct vhost_dev {
>      unsigned long long backend_features;
>      bool started;
>      bool log_enabled;
> -    vhost_log_chunk_t *log;
>      unsigned long long log_size;
>      Error *migration_blocker;
>      bool force;
> @@ -52,6 +58,7 @@ struct vhost_dev {
>      hwaddr mem_changed_end_addr;
>      const VhostOps *vhost_ops;
>      void *opaque;
> +    struct vhost_log *log;
>  };
>  
>  int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
> -- 
> 2.1.0