Re: [Qemu-devel] [PATCH v2 4/9] vhost: alloc shareable log

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Marc-André Lureau (marcandre.lureau@redhat.com) wrote:
> If the backend is of type VHOST_BACKEND_TYPE_USER, allocate
> shareable memory.
> 
> Note: vhost_log_get() can use a global "vhost_log" that can be shared by
> several vhost devices. We may want instead a common shareable log and a
> common non-shareable one.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  hw/virtio/vhost.c         | 38 +++++++++++++++++++++++++++++++-------
>  include/hw/virtio/vhost.h |  3 ++-
>  2 files changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 2712c6f..862e786 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -18,6 +18,7 @@
>  #include "qemu/atomic.h"
>  #include "qemu/range.h"
>  #include "qemu/error-report.h"
> +#include "qemu/memfd.h"
>  #include <linux/vhost.h>
>  #include "exec/address-spaces.h"
>  #include "hw/virtio/virtio-bus.h"
> @@ -286,20 +287,34 @@ static uint64_t vhost_get_log_size(struct vhost_dev *dev)
>      }
>      return log_size;
>  }
> -static struct vhost_log *vhost_log_alloc(uint64_t size)
> +
> +static struct vhost_log *vhost_log_alloc(uint64_t size, bool share)
>  {
> -    struct vhost_log *log = g_malloc0(sizeof *log + size * sizeof(*(log->log)));
> +    struct vhost_log *log;
> +    uint64_t logsize = size * sizeof(*(log->log));
> +    int fd = -1;
> +
> +    log = g_new0(struct vhost_log, 1);
> +    if (share) {
> +        log->log = qemu_memfd_alloc("vhost-log", logsize,
> +                                    F_SEAL_GROW|F_SEAL_SHRINK|F_SEAL_SEAL, &fd);
> +        memset(log->log, 0, logsize);

qemu_memfd_alloc can return NULL can't it - so that needs checking?

> +    } else {
> +        log->log = g_malloc0(logsize);

I know the old code also used g_malloc0, but if the log isn't 'small'
then g_try_malloc0 is possibly safer and properly return errors
if it can't be allocated.

Dave

> +    }
>  
>      log->size = size;
>      log->refcnt = 1;
> +    log->fd = fd;
>  
>      return log;
>  }
>  
> -static struct vhost_log *vhost_log_get(uint64_t size)
> +static struct vhost_log *vhost_log_get(uint64_t size, bool share)
>  {
> -    if (!vhost_log || vhost_log->size != size) {
> -        vhost_log = vhost_log_alloc(size);
> +    if (!vhost_log || vhost_log->size != size ||
> +        (share && vhost_log->fd == -1)) {
> +        vhost_log = vhost_log_alloc(size, share);
>      } else {
>          ++vhost_log->refcnt;
>      }
> @@ -324,13 +339,21 @@ static void vhost_log_put(struct vhost_dev *dev, bool sync)
>          if (vhost_log == log) {
>              vhost_log = NULL;
>          }
> +
> +        if (log->fd == -1) {
> +            g_free(log->log);
> +        } else {
> +            qemu_memfd_free(log->log, log->size * sizeof(*(log->log)),
> +                            log->fd);
> +        }
>          g_free(log);
>      }
>  }
>  
>  static inline void vhost_dev_log_resize(struct vhost_dev* dev, uint64_t size)
>  {
> -    struct vhost_log *log = vhost_log_get(size);
> +    bool share = dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER;
> +    struct vhost_log *log = vhost_log_get(size, share);
>      uint64_t log_base = (uintptr_t)log->log;
>      int r;
>  
> @@ -1136,9 +1159,10 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>  
>      if (hdev->log_enabled) {
>          uint64_t log_base;
> +        bool share = hdev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER;
>  
>          hdev->log_size = vhost_get_log_size(hdev);
> -        hdev->log = vhost_log_get(hdev->log_size);
> +        hdev->log = vhost_log_get(hdev->log_size, share);
>          log_base = (uintptr_t)hdev->log->log;
>          r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE,
>                                          hdev->log_size ? &log_base : NULL);
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 6467c73..ab1dcac 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -31,7 +31,8 @@ typedef unsigned long vhost_log_chunk_t;
>  struct vhost_log {
>      unsigned long long size;
>      int refcnt;
> -    vhost_log_chunk_t log[0];
> +    int fd;
> +    vhost_log_chunk_t *log;
>  };
>  
>  struct vhost_memory;
> -- 
> 2.4.3
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK