Re: [Qemu-devel] [PATCH V3] vhost: logs sharing

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

On Tue, May 26, 2015 at 01:54:09AM -0400, Jason Wang wrote:
> 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
> 
> Since each vhost device will allocate and use their private log, this
> could cause very huge unnecessary memory allocation. We can in fact
> avoid extra cost by sharing a single vhost log among all the vhost
> devices. This is feasible since:
> 
> - All vhost devices for a vm should see a consistent memory layout
>   (memory slots).
> - Vhost log were design to be shared, all access function were
>   synchronized.
> 
> So this patch tries to implement the sharing through
> 
> - Introducing a new vhost_log structure and refcnt it.
> - Using a global pointer of vhost_log structure to keep track the
>   current log used.
> - If there's no resize, next vhost device will just use this log and
>   increase the refcnt.
> - If resize happens, a new vhost_log structure will be allocated and
>   each vhost device will use the new log then drop the refcnt of old log.
> - The old log will be synced and freed when reference count drops to zero.
> 
> 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>

Almost there I think.

> ---
> Changes from V3:
> - only sync the old log on put
> Changes from V2:
> - rebase to HEAD
> - drop unused node field from vhost_log structure
> Changes from V1:
> - Drop the list of vhost log, instead, using a global pointer instead
> ---
>  hw/virtio/vhost.c         | 78 ++++++++++++++++++++++++++++++++++++-----------
>  include/hw/virtio/vhost.h |  8 ++++-
>  2 files changed, 68 insertions(+), 18 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 54851b7..fef28d9 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 +284,57 @@ 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);
> +    } else {
> +        ++vhost_log->refcnt;
> +    }
> +
> +    return vhost_log;
> +}
> +
> +static void vhost_log_put(struct vhost_dev *dev, bool sync)
> +{
> +    struct vhost_log *log = dev->log;
> +
> +    if (!log) {
> +        return;
> +    }
> +
> +    --log->refcnt;
> +    if (log->refcnt == 0) {
> +        /* Sync only the range covered by the old log */
> +        if (dev->log_size && sync) {
> +            vhost_log_sync_range(dev, 0, dev->log_size * VHOST_LOG_CHUNK - 1);
> +        }
> +        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);
> +    uint64_t log_base = (uintptr_t)log->log;
>      int r;
>  
> -    log = g_malloc0(size * sizeof *log);
> -    log_base = (uintptr_t)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, true);
>      dev->log = log;
>      dev->log_size = size;
>  }
> @@ -601,7 +640,7 @@ static int vhost_migration_log(MemoryListener *listener, int enable)
>          if (r < 0) {
>              return r;
>          }
> -        g_free(dev->log);
> +        vhost_log_put(dev, false);
>          dev->log = NULL;
>          dev->log_size = 0;
>      } else {
> @@ -1060,10 +1099,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>          uint64_t log_base;
>  
>          hdev->log_size = vhost_get_log_size(hdev);
> -        hdev->log = hdev->log_size ?
> -            g_malloc0(hdev->log_size * sizeof *hdev->log) : NULL;
> -        log_base = (uintptr_t)hdev->log;
> -        r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE, &log_base);
> +        if (hdev->log_size) {
> +            hdev->log = vhost_log_get(hdev->log_size);
> +        }

Why is this conditional on hdev->log_size?
What will the value be for log_size == 0?

> +        log_base = (uintptr_t)hdev->log->log;

especially since we de-reference it here.

> +        r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE,
> +                                        hdev->log_size ? &log_base : NULL);
>          if (r < 0) {
>              r = -errno;
>              goto fail_log;
> @@ -1072,6 +1113,9 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>  
>      return 0;
>  fail_log:
> +    if (hdev->log_size) {
> +        vhost_log_put(hdev, false);
> +    }
>  fail_vq:
>      while (--i >= 0) {
>          vhost_virtqueue_stop(hdev,
> @@ -1101,7 +1145,7 @@ 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, false);
>      hdev->log = NULL;
>      hdev->log_size = 0;
>  }

Why false? We better sync the dirty bitmap since log is getting
cleared.

And if it's always true, we can just drop this flag.


> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 8f04888..816a2e8 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -28,6 +28,12 @@ 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 {
> +    unsigned long long size;
> +    int refcnt;
> +    vhost_log_chunk_t log[0];
> +};
> +
>  struct vhost_memory;
>  struct vhost_dev {
>      MemoryListener memory_listener;
> @@ -43,7 +49,6 @@ 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 +57,7 @@ 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,
> -- 
> 1.8.3.1
>